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25 HP Briggs won't rev up to top rpm.

Posted by cedarghost none (My Page) on
Sat, Jun 4, 11 at 22:32

Motor 445777-0168-e1
The motor will start fine. Runs on low rpms (idle) fine. When I move the throttle to fast, the rpms go up a little, but not much.
So for what I have tried:
-Statically set governor
-Set Low idle governor settings
-Tried to set High-rpm governor settings, but the rpms didn't change. A gracious poster put up pictures of the correct location of the governor springs, so I know they are right.
-set valves. Again helped by a very gracious user on here, so I know they are right.
-checked compression. 150 on one side, 147'ish on the other.
-pulled flywheel and checked for a sheared key. Key was good.
-checked for spark. I have spark from both coils and plugs
-tried to test diodes in coil circuits, but my meter isn't working right.
-bypassed the fuel solenoid with a shutoff and disconnected the wires to it and took out the plunger.
-gave the carb a thorough cleaning.

The voltage regulator is bad and the clutch/brake safety isn't working. The safety issue is a problem that developed today, even though the mower hasn't been moved from my garage.
I don't know what to check next.....
A little background. I am starting a new post because my problem keeps changing as I eliminate symptoms.
I changed the sump gasket on a 445777-0168-e1 Briggs 25 hp. But I used RTV instead of a gasket. Nothing is rubbing that I can feel when I turn the flywheel, but I ordered gaskets anyway and will swap it out when it arrives.
In the meantime, after putting the motor back together, I have had several problems. Intermittent spark, gas shooting out of the carburetor, mis-adjusted vallves, you name it.


Follow-Up Postings:

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RE: 25 HP Briggs won't rev up to top rpm.

How should this thing run if I completely disconnected the governor? Maybe thats a place to start. There's no way I got the timing off without removing the cam or gears is there? Wouldn't timing cause backfires? Is there anything (besides compression) that could cause loss of power without backfiring, besides the governor?


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RE: 25 HP Briggs won't rev up to top rpm.

Uhhhhh! Hold on there pardner before you go off on yet another tangent. That there brake/clutch switch might in fact be working properly!!!!!
I don't believe you have revealed the make and model number of this here tractor in any of your posts.
MOST clutch/brake switches are 2 switches in 1 body, and the 2 switches control 2 different circuits for 2 different reasons (functions).
The reason your engine is dying when you let the pedal up is probably due to a malfunctioning (or defective) seat switch.
The logic in this circuit is that so long as the brake pedal is DOWN, the engine is permitted to run even when the seat is not occupied.
A defect in the seat switch (or its interlock system) is duplicating the conditions of an unoccupied seat, and in that state.......letting up the pedal kills the engine (because it's working properly).
So how about telling us (again if I missed it) the brand of tractor and the full, technical model number (from under the seat since you are going to look under there anyhow :^)


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RE: 25 HP Briggs won't rev up much.

Disconnecting the governor linkage, and subsequently operating the engine, is never advisable.
But if your earlier statement (which thread was that?) about the throttle plate standing wide open when you looked into the nether regions of the carb with the engine OFF and throttle control set to high speed would be proof enough that the throttle and governor are cooperating well.
Instead of disconnecting the governor, I will suggest that you use nothing more than finger tip pressure to move the governor arm and/or the governor springs that attach to the governor control swivel and see what effect that has on RPM.
With compression readings as you stated, your problem is definitely not due to lack of compression.
I'm thinking you might have a partially blocked fuel main jet.


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RE: 25 HP Briggs won't rev up to top rpm.

UPDATE...
I fixed the brake safety switch. After checking the wiring diagram, I saw that the fuel shutoff solenoid was on the "safety circuit". I fixed the plug and plugged it back it.
BUT the forward and reverse pedals do nothing, so tomorrow I'm going to check the manual and make sure I have the electric clutch mounted correctly after I look to see if the Voltage regulator needs to be in circuit for the clutch to work.
I'll let everyone know.


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RE: 250 HP Briggs won't rev up to top rpm.

Please share that wiring diagram. You have my e-mail address :^)


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RE: 25 HP Briggs won't rev up to top rpm.

I'll send you the schematic. It is from the manufacturer and if I read it correctly the fuel solenoid not being connected created an open when the brake was pressed. I had already jumpered the seat switch to eliminate it.
It is a Poulan Pro PR25PH48SCT.
I cleaned the carb in an ultrasonic cleaner filled with carb cleaner after complete disassembly. The main jet is clear and facing the side of the bowl away from the float hinge (per Birggs instructions).
I disconnected the governor linkage and it makes no difference in RPMS no matter which way i turn the governor rod. Same for when it is connected. Just so you know, the rod was turned all the way to the right (towards full throttle)after I statically set it.
If I leave the governor linkage attached, tractor at full throttle and push the linkage away from the full throttle position is idles down a hair, but not much and I assume that is because the throttle plates in the carb are closing a little.
Completely disconnecting the governor linkage does nothing. It runs at the same rpms that it started at unless i move the throttle linkage at the carb. Then it will die.
I appreciate the help. I know you guys have better things to do than put up with me. :)


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RE: 25 HP Briggs won't rev up to top rpm.

***"Completely disconnecting the governor linkage does nothing."***
Then you can shelve any concerns that the governor is playing a part in the malfunction.
The governor DOES NOT make the engine run fast, the governor keeps the engine from running too fast (but tries to maintain the RPM to a specific rate).

***"I know you guys have better things to do than put up with me. :) "***
We are very "tolerant", in fact, we are writing a book on tolerance :^)


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RE: 25 HP Briggs won't rev up to top rpm.

Thanks for the patience. I bought a book on small engines and am slowly working my way through it to better understand all this.
So the governor is out. I am going to go ahead and pull the motor tomorrow since my parts should be in on Tuesday. I'll check the timing when I have the crankshaft off, but I expect it would be backfiring if it was the timing wouldn't it?
I took the crankcase breather off after the carb today and the filter was clogged with oil. Hopefully they have one at the repair shop in town.
Does anyone know if the fuel nozzle gasket is supposed to have 6 holes in it? There are six holes underneath the gasket and nozzle, but my gasket only had 4 holes in it. 2 for the top two holes and 2 for the bottom. The middle set was covered, but the gasket was warped-looking at those holes and I couldn't tell if there were supposed to be holes there or not.
The instructions I followed said that air goes into the top two sets of holes to mix with fuel and the bottom set is where the air/fuel mixture goes into the venturi.
Here's a link with pictures: http://outdoorpowerinfo.com/repairs/briggs_intek_v-twin_carb2.asp
It sure seems like a carb problem at this point, but I didn't remove the carb when I pulled the engine, and I have pulled it and cleaned it to troubleshoot, in case it got dirt in it.
I'll let you guys know if I find anything out of the ordinary when I pull the crank cover again.
Here's a picture of the gasket I am talking about:
[IMG]http://i369.photobucket.com/albums/oo136/cedarghost/2011-06-05124621_Cynthiana_Kentucky_US.jpg[/IMG]
Not sure if that will work, so here's a direct link:
http://i369.photobucket.com/albums/oo136/cedarghost/2011-06-05124621_Cynthiana_Kentucky_US.jpg


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RE: 25 HP Briggs won't rev up to top rpm.

Hey Mownie, remember the "horn sound" I asked you about? It was coming from the crankcase breather, I think. If I blow through the tube that attaches to the carb, barely any air gets through (kind of sounds like and elk call). If I blow through the back side that faces down into the crankcase chamber, it makes the "horn sound" I was hearing. Also there was a little oil in the chamber and the filter was completely clogged with old oil. I don't smell any gas in the crankcase when I remove the dipstick.
Any of that affect rpms?


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RE: 25 HP Briggs won't rev up to top rpm.

The gasket you are holding between your fingers in the picture.......is that your old gasket from the carb? or the new gasket from the kit?
If that is your old gasket, there's a good chance that Nikki has made some changes to the design since the time the original manual was produced.
The middle holes that seem to be missing (according to your website manual and the Briggs service manual #273521) are explained (website manual) as air passages for main jet while the top holes are air passages for pilot jet.
Nikki may have combined the air passages into a single air feed for both main and pilot jets, thus the disappearance of the separate middle holes in the later (or earlier?) design.


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RE: 25 HP Briggs, any smoke?

Well, what you are now stating about the oil soaked air filter, the breather reed blowing like a horn, and the low RPM, and the fact that your ORIGINAL thread was about smoke from the exhaust.................still make me think that there is a chance a leaking head gasket is part of this scenario.


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RE: 25 HP Briggs won't rev up to top rpm.

That's the old one. I put it back in the carb for now. My kit should be in Tuesday with my gaskets. I wish you could see it closer. It is really warped on the back side (like the gasket material is being sucked into the air holes). My carb nozzle holes look exactly like the ones on the website.
This mower is older than what I was told. The date code shows 0311 for the first 4 numbers. That's November 2003, if I understand it correctly.
Thinking about putting the crankcase breather back on before I pull the motor and trying it, since it was so clogged. But, am I correct in thinking tha if it was clogged too bad it would force gas back past the pistons since it wasn't venting and it would be smoking a little?


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Any smoke?

No smoke now. If I check the head gasket by looking into the pushrod galley, am I looking for smoke being leaked there from the cylinder?
Does the horn sound coming from the breather valve mean it is bad?


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RE: 25 HP Briggs won't rev up to top rpm.

I mean crankcase gas, not fuel in reference to the crankcase breather being clogged. Wouldn't it force the combustion gas/smoke in the crankcase past the pistons instead of letting it back out the breather to be sucked into the intake valves? And wouldn't that cause a loss of power?
I'll check the head gaskets tomorrow.


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RE: 25 HP Briggs won't rev up to top rpm.

I can only guess that the "horn-like sound" coming from the breather is due to the reed valve or flapper valve vibrating at a certain resonant frequency as crankcase pressure is vented out.
While "blow-by gases" are able to escape past piston rings with relative ease (because the compression and combustion pressures are so high), getting crankcase gases to migrate in the opposite direction (from crankcase to combustion chamber) would be much more difficult.
Compression pressures can vary but generally will be in a range from around 135 PSI in some engines to as high as 175 PSI in other designs.
Pressures that high have relatively little trouble escaping through the labyrinthine route of the piston rings from combustion chamber to crankcase.
COMBUSTION pressures are even higher than compression pressures (combustion pressures can only be measured in laboratory engines).
Plugging up all the possible vent provisions on an engine will result in oil seals "turning inside out" long before the crankcase pressure rises enough to force a significant volume of gases up through the piston rings and into the combustion chamber.
Having stated all that, the crankcase breather is only supposed to be expected to handle a very small volume of actual "blow-by gases", plus the small amount of PRESSURIZATION caused by a descending piston (pistons cause increase in crankcase pressure on the down stroke).

A failed head gasket can cause the breather to be overwhelmed by the volume of compression/combustion gases that enter the crankcase through the breached gasket.


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RE: 25 HP Briggs won't rev up to top rpm.

Pulled the heads and both headgaskets are good. One has a small piece missing on the backside of one of the bolts, but it shouldn't cause a leak. I will post pictures when I can get to a PC. The valves had a LOT of carbon build up on them. I wire brushed them the best I could. The piston heads didn't look too bad at all. The cylinders were really smooth from what 8 could see of them.
I think you're narrowing it down.
not a head gasket and not the governor.
I guess that leaves the carb or crankshaft rubbing, which will be eliminated. When I replace the gasket.
or the coils may be firing intermittently, I guess.


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RE: 25 HP Briggs won't rev up to top rpm.

If the intake valves are absolutely "packed" with carbon/gunk, that could reduce the ability of the engine to "breathe" at higher RPM.
But if that is the case, the previous owner would also have had the same problem.
You can test the coils for intermittent firing by using the Briggs spark tester shown in the manual.
The Briggs tester permits you to watch the spark producing performance of the ignition while the engine is actually operating.


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Pictures of heads

Here's a couple pictures of the heads:
http://i369.photobucket.com/albums/oo136/cedarghost/2011-06-06100515_Cynthiana_Kentucky_US.jpg
http://i369.photobucket.com/albums/oo136/cedarghost/2011-06-06103406_Cynthiana_Kentucky_US.jpg
You can see the spot of the gasket that is missing in the second picture at the top bolt.
Here is a picture of one of the heads with the valve seat showing:
http://i369.photobucket.com/albums/oo136/cedarghost/2011-06-06095826_Cynthiana_Kentucky_US.jpg
Thanks for the help.


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RE: 25 HP Briggs won't rev up to top rpm.

The picture of the head gasket/head looks pretty good and normal.
The picture of the head combustion chamber looks pretty nasty.
What does it look like inside the intake port on the heads?


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RE: 25 HP Briggs won't rev up to top rpm.

Nasty. I'm headed home now with a new set of wire brushes, to clean her all up. Also found an ignition tester at Harbor Freight for $4! My torque wrench only goes to 150 and theads torque higher than that. Looks like a new torque wrench is in order. How critical is it, or what's the tolerance on the exact poundage? Chance of warping heads I assume, if they're too tight? Danged carb kit won't be here till Friday. My gaskets will be here tomorrow.
ill put up pics of the ports before I clean them.


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RE: 25 HP Briggs won't rev up to top rpm.

There is a better way to post pictures on this forum.

In Photobucket click and copy the 'HTML code' for your picture. Then, paste that 'HTML code' into your post. Pictures will appear inline in your post. Confirm that you have done it correctly by using 'Preview Message'....


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RE: 25 HP Briggs won't rev up to top rpm.

Having problems uploading the picture on dial up. The intake valve was mostly sooty-looking. It definitely had some carbon buildup! I went ahead and took all the valves out and cleaned them and the valve cylinders really well with a soft wire brush in a sink of sudsy water, then dried everything with a hair dryer. They look good now. I figured I might as well, since I had the parts coming anyway.
This time when I pulled the crankcase, the cam slid out with the bottom half. I wonder if it did that before and I slid it back in a notch off on the timing...
I inspected the pistons the best I could, but I don't have a gauge to check, so it's just a visual inspection. I don't see any crosshatching in the cylinder walls, but it had good compression anyway, so I'm not too worried.
Here's my checklist for reassembly, please let me know if I missed anything.
-Check timing marks
-install sump drive shaft
-Dry install new crankcase gasket with bottom pan and check crankshaft end play. Adjust with gasket as needed and install. Should I use RTV on both sides of the new gasket? I'm thinking not.
-attach governor linkage and carb linkage and attach springs BEFORE I install the carb
-adjust static governor settings (I didn't see anything in the manual about making sure the governor was in a certain position or anything before installing the sump. It looks like the camshaft gear just spins it causing the throw-out mechanism to pull the shaft open at the top.
-install carb
-install heads and torque to spec (how critical is this, before I buy a new torque wrench?) Mine only goes to 150.
-install rods and rockers
-adjust valves to .005 at 1/4" past TDC
-install new valve cover gaskets
-attach belts and clutch assembly (wish the manual had a better picture of the clutch assembly position)
-put in new oil and filter (the new oil I put in when I pulled the motor last week was black as soot, even though the motor had probably only ran a couple hours. I assume that was everything being cleaned out.
-reattach all wiring, heatshields, etc.
-turn over without plugs for about 15 seconds to let everything lube up
-check for fire from coils with ignition tester, then check for fire from plugs.
-fire it up and pray....
-It may or may not run right depending on the carb. Some of the gaskets look questionable (warped), but I'll be rebuilding the carb next weekend anyway.
I don't think I am missing everything, but I think I will need a new torque wrench. Both the crankcase bolts and the head bolts are supposed to be torqued higher than mine goes.
Wish me luck!


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RE: 25 HP Briggs won't rev up to top rpm.

"Should I use RTV on both sides of the new gasket? I'm thinking not. "

I'm thinking NOT not!

I have used RTV twice on the same style engine with good results (no leaks). Here is how:

1. Put a very light application of RTV blue gasketmaker on the mating surface of the sump cover.

2. Apply the sump gasket to the 'buttered-up' sump cover and press in place firmly, all the way around. Be sure all holes are clear of RTV.

3. Allow the sump cover/gasket assembly to dry overnight.

4. Apply a very light application of RTV blue gasketmaker to the mating surface of the block.

(Here you might want to pause and take one more look to be sure the cam and crank timing marks are lined up! It will be too late once you bolt on the cover. Maybe even take a picture to convince yourself later that you did it correctly...)

5. Install the sump cover/gasket assembly on the block, insert bolts and torque to spec in sequence.

This procedure eliminates any possibility of the gasket sliding out during assembly and the RTV seals very well. This is what it will look like from the outside when you are finished:

Photobucket


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RE: 25 HP Briggs won't rev up to top rpm.

Will do! You guys are great! I need to mow, but no use rushing it with this much time in. My neighbor said I could use his John Deere and 4 foot woods mower to knock down the front 3 acres if it gets too high...
i've learned so much during this process. I appreciate the help and the knowledge you guys have and are willing to share.


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Timing

Anyone know a good resource I can look at to better understand timing on these engines? I'm wondering if the timing was off a hairfrom the last time I took the crankcase off. Because it was an immediate change to low rpms as soon as I remounted the motor.


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RE: 25 HP Briggs won't rev up to top rpm.

Chapter 13 of your manual is all you need to know on aligning the timing marks of the crankshaft and the camshaft gearing.
Ignition timing on Briggs is driven off the flywheel/crankshaft. A spark is generated each time the ignition magnet passes the magneto coil. This means that you have a spark during the last few degrees of the compression stroke (prelude to the power stroke), and.....you have a spark during the last few degrees of the exhaust stroke (prelude to intake stroke).
The spark that occurs during the exhaust stroke does nothing at all for the engine, thus its appropriate title of "wasted spark ignition".
Wasted spark ignition is a characteristic of ignition systems that are driven off the crankshaft.
Ignition systems that are driven off the camshaft do not have a wasted spark because the camshaft turns precisely 1/2 of crankshaft speed, so the spark always occurs during the last part of the compression stroke, and none on exhaust stroke.

It is vital that you get a new torque wrench to properly tighten the head bolts.
Uneven torquing can lead to warping the head or even the block.


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RE: 25 HP Briggs won't rev up to top rpm.

  • Posted by baymee LehighValleyPA (My Page) on
    Tue, Jun 7, 11 at 5:54

You said you need a new torque wrench. Is your current model 150 foot pounds max or inch pounds? Heads are torqued in inch pounds, but can be converted to foot pounds. A recent head job on a Kohler 23HP required a final torque of 31 foot pounds or 370 inch pounds.


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RE: 25 HP Briggs won't rev up to top rpm.

My Torque wrench is ft. lbs. The torque for the heads are 220 in. lbs. so that would be 18.33 ft. lbs. so I'm good to go!
The crankcase cover is supposed to be torqued to 200 in. lbs. so 16.7 in. lbs. Again, good to go!
Thanks!
Thanks for the timing information Mownie. There was a chapter about it in the book I bought as well that explained the different ways timing and spark are achieved on 2 cycle, 4 cycle, how it is affected by spark gap and point settings, etc.


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RE: 25 HP Briggs won't rev up to top rpm.

The wife just called and my parts are in. Too bad I work until 10 tonight.....I'll be up early in the morning to start on her but I may not get her back in the mower until Friday if I use RTV on the gasket. I may use Indian Head as someone suggested on my other thread. I know my dad swears by that stuff.
I'll keep you posted.


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RE: 25 HP Briggs won't rev up to top rpm.

Actually you would be better off using inch pound torque wrench? why? torque wrenches are calibrated to Plus or mini 1. So would you rater be foot off or inch (12 pounds vs. 1)?

also the lower 20% of any breakaway torque wrench in not too accurate unless it is specificlly cal. in that range which makes it off at the other end of the range. SO you would want to have torque wrench that your settings would be useable in the upper 80% range. ALso, breakaway torque wrenches should be cycled at least 4 times before use especially if they were not used for long peroid of time. This lubricated the spring in theroy (not mine, in manuals on torque wrenches.)

Does it really matter? nope just workig on lawn equipment, not like it's going to drop out of the sky!


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RE: 25 HP Briggs won't rev up to top rpm.

"Does it really matter? nope just workig on lawn equipment, not like it's going to drop out of the sky!"
You haven't seen what I've been through on this tractor! lol I have been taken to school by you guys on here, and I appreciate it. This is going to be a new hobby for my 16 year old an myself. We both love it. In fact, he managed to talk my father-in-law out of an 8 hp Kohler that he replaced with a new one on his roto-tiller and is wanting to build a go cart with it....
Any way, what you say makes sense. I'll swing by Harbor Freight tomorrow and see what they have. Thanks!


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RE: 25 HP Briggs won't rev up to top rpm.

  • Posted by baymee LehighValleyPA (My Page) on
    Tue, Jun 7, 11 at 21:42

I agree. If you do this often, you should certainly invest in an inch pound wrench.


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RE: 25 HP Briggs won't rev up to top rpm.

Solved!!! The timing must have been off a bit. Got her all back together and just mowed 2 acres faster and smoother than I ever have been able to. The governor still needs adjusted, but I don't have a timing light. But it never bogs down even in deep grass. Starts immediately.
if I try to run it on high throttle it will pop through the carb a little in really deep grass. Keep it on half and it sounds like what it should on high. I'll get a light tomorrow.
thank you guys SO MUCH, especially Mownie for your patience. Its a very satisfying experience when you first mow with it after doing all that work!
now my son is ready to tear into this 8 hp Kohler.....
wish us luck!


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RE: 25 HP Briggs won't rev up to top rpm.

Solved!!! The timing must have been off a bit. Got her all back together and just mowed 2 acres faster and smoother than I ever have been able to. The governor still needs adjusted, but I don't have a timing light. But it never bogs down even in deep grass. Starts immediately.
if I try to run it on high throttle it will pop through the carb a little in really deep grass. Keep it on half and it sounds like what it should on high. I'll get a light tomorrow.
thank you guys SO MUCH, especially Mownie for your patience. Its a very satisfying experience when you first mow with it after doing all that work!
now my son is ready to tear into this 8 hp Kohler.....
wish us luck!


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RE: 25 HP Briggs won't rev up to top rpm.

Hopefully the carb kit will fix the backfiring. It got worse after I mowed some more, the engine would almost die when I attempted to drive with the blades engaged. Think I found the culprit though. I drove it on back to the house and when I got ready to shut it down, it started smoking REALLY bad from the BOTTOM of the engine.
First thought...I THREW A ROD....So I pulled the valve covers and everything was good. Turned the engine over, started right up and the valves were doing their thing. Then I smelled the smoke and it was burning rubber. I shut it down and looked, and the drive belt is in a bind somehow and rubbing on the pulley. WHEW!!! I wonder if that is enough of a load to cause the engine to lose power....guess we will find out.
I went ahead and made the low rpm setting on the governor (with the engine off) and started it back up and the idle is a lot better. It was way too high.
Then I called it a day. The heat index here in Kentucky is over 100 today and I have been at it since this morning.


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RE: 25 HP Briggs won't rev up to top rpm.

  • Posted by baymee LehighValleyPA (My Page) on
    Wed, Jun 8, 11 at 20:18

You don't need a timing light anymore. To adjust the governor, it's as simple as moving the throttle lever to wide open position, loosening a nut, and turning the screw either CW or CCW. You'll have to research that one.

How did you know the timing was off a bit? What did you find?


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RE: 25 HP Briggs won't rev up to top rpm.

It was luck really. When I pulled the engine this time, the camshaft slid out a little bit. I think I remember it doing that last time and I just pushed it back it, paying no attention to the dots....
All I did this time was put the proper crankcase gasket on it, cleaned and lapped the valves, cleaned the heads, put it back together, readjusted the valves and it runs like a scalded dog...
It was running decent before I pulled the engine the first time, but after I put it back on, it was hard to start and wouldn't reach full rpm. Nothing I did changed that until I pulled the motor again and made sure the timing dots were lined up.
What you mentioned above is "statically" setting the governor. I have done that. But now I need to rebend the tangs as I did bent them when it was running rough and now it idles too high and the top rpm sounds way too high. Not like it's gonna blow, but it's strokin'!!! It can't have that much power...
I already set the low-idle rpm by bending the tang and it's good now. Now I just have to figure out why it's popping through the carb when the blades are engaged and under load. I just pulled the plugs and one cylinder was flooded pretty bad, but first things first, my drive belt is in a bind (started smoking on the pulley), so maybe that and the added stress on deep grass and engaged blades may be my only problem.
Friday I will check to make sure both plugs are firing right. I think they will be and what I will find is the carb has issues. I have a sneaking suspicion my nozzle gasket is bad and/or the o-ring on the main jet or fuel shutoff solenoid is allowing more gas to leak into the carb than it should.
This has been one heck of a ride!!!


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RE: 25 HP Briggs won't rev up to top rpm.

The popping back through the carb may be due to the engine not getting ENOUGH fuel. If it was getting TOO MUCH fuel you would probably be telling us that you see some black smoke out the exhaust and the fumes are really foul.
Hopefully it is only due to insufficient fuel delivery.
I shudder to think it might be due to too high RPM floating some valve springs.


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RE: 25 HP Briggs won't rev up to top rpm.

  • Posted by baymee LehighValleyPA (My Page) on
    Thu, Jun 9, 11 at 5:46

Maybe you can borrow an RPM meter from somebody. You need an experienced ear to set RPM without a meter and even then you're not always accurate.

There is a small round device with a vibrating spring that is pretty good at RPM setting. Not too expensive.


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RE: 25 HP Briggs won't rev up to top rpm.

I've got a meter. You put a piece of reflective tape on the flywheel and it measures off that. I'll let you guys know what the rpms are after I measure tomorrow.
I don't think the rpms are THAT high Mownie, but it did sound a little high, so I was running it at about 3/4 throttle and it sounded around what it sounded like before I did all this work.


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RE: 25 HP Briggs won't rev up to top rpm.

Word of caution here.
If you plan to remove the flywheel/blower cover in order to apply the tape to the flywheel...........and you will "shoot the strobe" at the moving target with the flywheel cover removed..................keep your actual engine run time to less than a minute.
With the flywheel cover removed there is no real air flow washing down on the heads and cylinders, and they will heat up very quick at max RPM and no air flow.
Also, the further out on the flywheel (from the crankshaft center) you are able to place the reflector target.....the easier it will be to find the synchronous moment using the adjuster knob.


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RE: 25 HP Briggs won't rev up to top rpm.

Great....I ran it for about 5 to 10 minutes without the shroud before I used it yesterday. Hopefully my backfiring isn't due to a warped head or valve stem or something...
Learning things the hard way sucks...
The backfiring was very minimal as long as I kept the throttle about 3/4's and was non-existent in "normal" grass. After she had ran for a couple hours, the backfiring got worse when in deep grass, so I just parked it and that's when I discovered the drive belt was getting in a bind.
The more I learn, the more I realize I need to learn.
As for the rpms, I will let you know how that goes tomorrow.


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RE: 25 HP Briggs won't rev up to top rpm.

Causes for backfiring through the carb:
Improper intake valve adjustment
Improper intake valve seating
Carburetor lean
Is that about right? Makes me wonder if I got the valves lapped good enough. and if I got them back in the same cylinder head they came from...


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RE: 25 HP Briggs won't rev up to top rpm.

I would say you can eliminate valve adjustment or valve lapping as a cause. It's just not that likely to be the cause.
Wait until you get the RPM right before you pursue the cough back issue.


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RE: 25 HP Briggs won't rev up to top rpm.

I will wait. I have at least learned not to go off on tangents until I eliminate know problems. Mostly thanks to you. Haha!


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RE: 25 HP Briggs won't rev up to top rpm.

I just thought about this...when I soaked the carb in carb cleaner, it still had most of the o-rings in it...
Come on carb kit!!!


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RE: 25 HP Briggs won't rev up to top rpm.

Low idles set to 1775 rpm, top is around 3650. No backfiring, BUT the deck is off of it right now. The drive belt had split part of itself off and was wrapped around the pulley on rear end and jamming it up. SO, I can't test it under load. It does feel like it isn't getting gas quick enough, so I bet it has carb issues. Unfortunately, my carb kit is on backorder. After all the negative stuff I have read about this carb on the internet, I am wonder if I would be better off to pop $140 for a new carb.
Found another problem too. There are no intake gaskets where the intake ports from the carb connect to the intake manifolds, so It's possible I have air coming in there. I may just make some gaskets for that and see if it makes a difference, but I can't try anything until I get the drive belt and mower deck back on it.


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RE: 25 HP Briggs won't rev up to top rpm.

There are no gaskets where the intake manifold meets the head because an O-ring is used to seal each flange. Briggs part # 697891. Requires 2 per engine.
I am thoroughly enjoying the wiring diagram...........:^)


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RE: 25 HP Briggs won't rev up to top rpm.

Carb kit installed. Mower is running like a champ. Doesn't even make a full revolution before it starts. Runs great under load as well.
Sorry. Forgot to send the wiring schematic for the tractor. I just did.
The fuel shutoff solenoid isn't part of the safety circuit. I was looking at the regular solenoid...which isn't part of the safety circuit either, but completes the circuit to the starter when the brake pedal is down, provided the PTO is in the disengaged position. Which leaves me to wonder why the brake pedal problem was solved by plugging in the fuel shutoff solenoid. Only thing I can figure is something was loose intermittently because that's ALL I did. If it was intermittent, I guess I'll see that problem again.
I still have a couple things to do. I noticed a little smoke when mowing (not much). It is coming from one of the exhaust pipes at the head. The gasket only has one bolt going through it. The other side of the gasket is slid out from under the exhaust pipe and there is a little gap there as a result.
Also, one of the bolts that holds the carb manifold to the head is partially stripped and will need tapped out to get it as tight as it needs to be.
One of the sump cover bolts had a little oil on it too, but that could have come from a valve cover, so I cleaned it up and am withholding judgment till I mow with it again.
Thanks for all the help.
Now I have a weedeater to fix and we will be taking a look at that Khoeler soon.


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RE: 25 HP Briggs won't rev up to top rpm.

"***one of the bolts that holds the carb manifold to the head is partially stripped and will need tapped out to get it as tight as it needs to be."***
The proper fix for that will be to get a "Heli-coil" thread repair kit. They come as a kit for a specific size of threaded hole that permits the original size bolt to be used after the repair is done.
Heli-coil is just one brand of thread repair systems that allow a repair to stripped threaded holes and usually, the repaired hole is much stronger than the OEM threads.....especially where the base metal threaded hole is in aluminum.


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RE: 25 HP Briggs won't rev up to top rpm.

Thanks for the tip. I'll order the kit. As you know per our email, I think I have burned the starter up by doing something not so smart....
I left a diode with one connector lead broken in the PTO switch connector, and engaged the PTO while the engine was running. So essentially it was out of circuit. Yes, I knew better. Now I definitely know better as I think my starter is shot.
The solenoid is good as I have 12v when I put one meter lead to ground and put the red lead to the other 12v lead on the solenoid when the key is "on".
Battery is not an issue as I had it jumped from my car. But the starter wouldn't spin up. I checked, and I do have @ 12v to the starter when the key is switched to start.
So I am assuming the current feeding back from the coil in the PTO fed back to the starter. What is racking my brain is "how"???
Mownie have you looked at that schematic? It looks like brake down, PTO disengaged, the coil in the solenoid closes the circuit, juice goes to the starter and away we go. If the starter is good...
Now we release the pedal (up position, and we engage the PTO switch. The connection that energized the coil in the solenoid, no longer does that. Another connection in the PTO switch now sends 12v to the coil in the clutch, magnetizes the coil which closes the gear(? didn't look close to see what is in there) to allow the blades too engage. Now we are mowing.
But....the diode that shunts the current coming back from the EMF created by the coil is no longer there, allowing current back into either the ignition or ground. Am I looking at that right? I don't see how the current could get back to the starter. Also I don't know which wire that current generated by the PTO coil will be coming out of (there are two leads the same color coming from the PTO coil).
It is grounded through the OPR and the 12v comes from the PTO switch in the engaged position, provided all safety circuits are complete. I am not seeing how current could get back could get back to the starter and fry it. I guess if the current ran to ground it could reach the starter, but it seems like that would kill the whole circuit, shutting the mower off.
I am going to have to dust off my electronics books and do some reading on coils and short circuits.
I took the starter apart and all brush wires have continuity from the leads to the magnets, and I saw that the rebuild kit for this starter is just brushes. I inspected the windings the best I could and didn't see any breaks. But the starter doesn't spin very easily at all. Feels pretty rough. And if it doesn't kick out, it doesn't kick out.
The reason I am analyzing this so much is, if I buy a new starter or rebuild this one, and put a new diode in, I want to make sure the new starter doesn't get burned up. I can easily connect the diode in the proper polarized connection.
But if the current somehow shorts through ground to the starter instead of being shunted through the diode...well you get the picture. I am probably forgetting some simple electronics principle as I am pretty rusty.


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RE: 25 HP Briggs won't rev up to top rpm.

After thinking about it, IF the PTO sends voltage spikes to ground, everything that is grounded on the tractor would get those spikes.....including the starter.
So who knows what all burned out. Luckily the voltage regulator was out of circuit, so the alternator should be ok. I'd be surprised if the PTO switch or OPR were good though.
I should be able to test them with the meter to find out since I have the schematic for them.
What I can't wrap my mind around is how to test which lead from the PTO coil needs to be positive and which needs to be negative. If I remember correctly, a coil just needs current. Direction of current flow doesn't matter. In that case, I test the PTO switch, OPR and install a new starter and rebuild the old one and I should be good to go.


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RE: 25 HP Briggs won't rev up to top rpm.

The voltage spike that is COMING BACKWARD out of the PTO coil is merely trying to find its way to "ground". That the spike "goes to ground" does no harm in itself (and it DOES NOT flow to other components once it reaches ground).
Where the harm is done is when the spike jumps (arcs) across the air gap separating the (now open) PTO switch contacts.
That is what would happen if there was no diode in place to shunt the EMF spike to ground.
Rest assured that though the spike is energetic enough to cause some arcing and burn up the PTO switch contacts in a relatively short time................there is no where near enough energy in that spike to harm the starter or the starter solenoid.
The voltage spike that wants to flow somewhere is actually akin to capacitor discharge in that there are really a series of diminishing oscillations of EMF as the coil electromagnetic field collapses back through the same windings that create the field when the coil is energized.
At some points in this event, the EMF is flowing in the normal direction, at other points in the event, the EMF is flowing in reversed polarity. Essentially the self induced spike is a rapidly fading alternating current.
And the entire event probably lasts less than one thousandth of a hiccup!
As to the comment about the regulator. Here again, the regulator can handle the paltry amount of EMF that could possibly be fed to it from the PTO coil. The regulator after all has its own diodes and is designed and built to handle the alternating current fed to it from the stator coils.
Whatever problems your starter has did not come from an errant diode.

Check out the starter per chapter 6 of the manual.


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RE: 25 HP Briggs won't rev up to top rpm.

Here is a little more information that might help clarify the purpose of the diode across the PTO coil (paraphrased from a Wikipedia article):

The functional name given to the diode is a 'snubber'.

Snubbers are frequently used in electrical systems with an inductive load (PTO coil) where the sudden interruption of current flow often leads to a sharp rise in voltage across the device creating the interruption (the PTO switch). This sharp rise in voltage is a transient and can damage and lead to failure of the controlling device (PTO switch).

When the current flowing is DC, a simple rectifier diode is often employed as a snubber. The snubber diode is wired in parallel with an inductive load (such as the PTO coil). The diode is installed so that it does not conduct under normal conditions. When current to the inductive load (PTO coil) is rapidly interrupted (by the PTO switch), a large voltage spike would be produced in the reverse direction (as the inductor (PTO coil) attempts to keep current flowing in the circuit). This spike is known as an "inductive kick". Placing the snubber diode in inverse parallel with the inductive load (PTO coil) allows the current from the inductor to flow through the diode rather than through the switching element (PTO switch), dissipating the energy stored in the inductive load over the series resistance of the inductor (PTO coil) and the (usually much smaller) resistance of the diode.

So, this inductive kick does not feed back into ground or into any other part of the engine such as the starter (as mownie correctly mentioned). The potentially damaging energy is dissipated in the series resistance of the diode and the PTO coil.


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RE: 25 HP Briggs won't rev up to top rpm.

***"The potentially damaging energy is dissipated in the series resistance of the diode and the PTO coil."***
That is correct, however it is also correct that the dissipating current and electron flow DO USE a portion of the chassis ground during that event in order to get back into the PTO coil windings.

Photobucket


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RE: 25 HP Briggs won't rev up to top rpm.

Here is an intuitive way to understand the job of the 'snubber' diode, that I find easier to relate to.

The only thing you have to remember about inductors: The CURRENT in an inductor (PTO coil) cannot not change instantly, even if the PTO switch is opened instantly.

So, a simple way to look at mownie's nice colored arrows is:

1. The PTO is engaged...current, from the battery, is flowing per the RED arrows through the PTO coil.

2. The PTO switch is opened instantly. The circuit is broken so all the RED arrows must disappear but....

3. The current represented by the RED arrow in the PTO coil cannot change instantly. There is no longer the normal RED arrow path for the current, because the PTO switch is open. Something has to give.....

4. Thanks to the diode, there is a path for the RED current to 'go'. It just continues in the same RED direction through the PTO coil then through the diode, represented by the BLUE arrow. It goes 'around in circles' until the energy dissipates as heat in the PTO coil resistance.

If there is no diode or the diode is defective, when the PTO switch is opened, the current still cannot change instantly, so the RED arrow PTO coil current WILL continue to flow by momentarily arching across the open PTO switch contacts (eventually destroying the switch).


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RE: 25 HP Briggs won't rev up to top rpm.

Wow. That's an awesome explanation. That schematic looks vaguely familiar too. :)
I got the starter on today. Now I am waiting for the Regulator.
After I put the starter on, I turned the motor by hand a couple of turns, and I felt a slight "bump" in the bottom of the motor. It felt like the pulley was hitting the bottom of the motor slightly, or the crankshaft was hitting the pan. Is that normal? I checked the crankshaft endplay when I installed the sump gasket, and turning it over by hand at that point, there was no bump.
I let my son put the pulley and clutch on. I think I better ask him if he torqued it. I have a feeling he got it too tight.


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RE: 25 HP Briggs won't rev up to top rpm.

If you are impatient and don't want to wait for the regulator...the engine should start just fine and run without the regulator. Once running, it will not charge the battery. So, don't turn on the PTO for very long as it will fairly quickly discharge the battery (watch the ammeter to see the discharge).


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RE: 25 HP Briggs won't rev up to top rpm.

Well, the bolt broke off in the crankshaft when I went to loosen the pulley, which was way too tight. So, I pulled the motor. That only took about 20 minutes this time, lol.
I drilled the bolt out and my extractor broke off in it. I had it heated up pretty good too.
I called the machine shop and they said they would laser cut it out and re-tap it for $25. So I will be taking it in to them tomorrow. In the meantime, I think the top seal may be leaking a little bit. There was a little oil pooled up there and it looked like it had been trickling down to the breather. I'll post a picture when I can get it to upload.
I guess I might as well replace the seals since I will have the crank out. Is there anything else you guys would replace while it was out?


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RE: 25 HP Briggs won't rev up to top rpm.

Ok. I got the crankshaft machined and retapped. Put everything back together and the knock i heard when turning the motor by hand coming from the bottom of the motor is still there. I was thinking maybe the flywheel wasn't tight enough and I did have a tough time torquing it to 150 ft. lbs.
I was thinking this because if I grab both sides of the flywheel and pull up on it, and turn the engine like that, there is no knocking. Could it be the crankshaft endplay? I didn't get a gasket set when it came in. There was only one gasket.
I hit the top of the crank pretty hard (I had a piece of wood on it) when I was taking the flywheel off. Could I have moved the seals down?
I turned the motor over for a few seconds with the key and the plugs out and it knocks about every revolution of the flywheel.
Should I start a new thread on this?


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RE: 25 HP Briggs won't rev up to top rpm.

  • Posted by baymee LehighValleyPA (My Page) on
    Wed, Jun 22, 11 at 5:39

150 foot lbs. sounds very high to me. Is that what the manual says?


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RE: 25 HP Briggs won't rev up to top rpm.

Yeah, I thought so too, but that's what the manual calls for. Found the source of my "bump" noise. The new starter I put on had a tab on top that had to be cut off because it was hitting the flywheel. I completely dissassembled the motor and checked the bearings, connecting rods to make sure they weren't bent, the whole nine. Put her back together and only got the bump noise after i installed the flywheel. I had my son turn it over by hand while I looked at everything and noticed the starter moving a little. There it was...
Started great, mowed great, then started smoking like crazy. Lol. I looked underneath and there was oil coming from the bottom of one of the heads. Took the heads off and about 3/4" of one of the gaskets was missing on the bottom. Easy fix.
Man this has been an EXTREME learning experience for both me and my son.


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