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newish Briggs backfires and won't start

Posted by LRM2 5 (My Page) on
Fri, Sep 20, 13 at 21:20

Hi all,
I've had some issues with an older Craftsman riding mower. (http://forums2.gardenweb.com/forums/load/tractor/msg0413270418571.html) I installed a new motor this spring and it worked fine all summer until now. I don't have the exact model number in front of me, but it is a 13.5HP Intek, series 21B800.
A couple of days ago, I had mowed for 20 minutes or so and the mower slowed to a rough idle. I thought I had run out of gas, so I stopped and turned off the key. There was still some gas in the tank, but I refilled it anyway. Struggled to restart, and it never got going better than the rough idle, and even that for less than a minute. Now it won't start at all, but occasionally backfires pretty strongly. Here's what I've done:
drained and refilled gas
replaced gas lines
removed and cleaned carb
replaced overfire solenoid with plug (I don't think that was teh problem, but wanted to rule out the possibility)
replaced spark plug
checked spark with spark tester

It seems to have good spark, and gas seems to be getting to and thru the carb - in fact, the spark plug is soaked and it seems like gas is getting all the way to the muffler, where it sometimes ignites. I pulled the valve cover, and the top valve goes in and out when I turn the flywheel by hand, but the bottom valve does not. I can push in the bottom valve by hand, but the push rod does not move in and out at all when I rotate the flywheel.
What should I be looking at to try next?
Thanks,
Lewis


Follow-Up Postings:

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RE: newish Briggs backfires and won't start

New cam shaft.


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RE: newish Briggs backfires and won't start

Thanks for the quick reply, Bill. Sorry to sound dumb, but would that be listed as a "crankshaft" on the parts list? Like here:
http://www.partstree.com/parts/?lc=briggs_and_stratton&mn=21D807-1546-G1&dn=102660003
Lewis


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RE: newish Briggs backfires and won't start

OK, I think I see now. In that link, the crankshaft is part #16, and the camshaft is #46. On a sliding scale, with pulling and replacing this engine being maybe a 3, and a carb rebuild with a couple of welch plugs being maybe a 4, how difficult is replacing a camshaft?


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RE: newish Briggs backfires and won't start

Sorry to keep replying to my own post, but I wanted to add the specific model number and type:
21B807 0884 B1


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RE: newish Briggs backfires and won't start

Sliding scale estimate of difficulty for you, about 8 or 9.


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RE: newish Briggs backfires and won't start

Kind of hard to estimate how difficult something is for someone else, since our brains all work differently, have different experience/mechanical ability & available "proper" tools.

I build bicycle wheels for fun. Others will never be able to do it, but can probably do something I can't. Like good penmanship.


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RE: newish Briggs backfires and won't start

Thanks for the replies. Maybe somewhere in the "challenging, but not impossible" range.
Is there any way to generalize about what causes damage to a camshaft? I'd hate to replace the camshaft only to have the new one fail because I missed the root cause of the issue.
This engine has had a pretty short and unstressful life so far, and it died pretty gently -- nothing like the crashing sounds of a connecting rod breaking. What kind of collateral damage might I find? Oil slinger, maybe?


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RE: newish Briggs backfires and won't start

You will likely not find any collateral damage because the condition is due to Briggs having done a rather poor job of heat treatment/hardening and possible poor choices of alloy material that all have to do with the camshaft itself.
Other factors affecting general health of engine components would be the quality of engine lube oil, whether the correct grade/specification of oil was used, and how often the oil was changed. Whether or not the engine oil level was maintained carefully, and if contamination/dilution of the lube oil by a defective carburetor float valve took place regularly.............could also be a set of factors.
If you decide to undertake this operation.......you need to use your digital camera and document every step of your dis-assembly so you can review later as a guide to how things were in place. Trust me on that! You will be foggy at some point when you start going back together.


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RE: newish Briggs backfires and won't start

I have a different theory on cams wiping out lobes.
Our new, modern motor oils.
Older oils used zinc & phosphorus as anti wear additives. Those happen to "poison" catalytic converters. and have been replaced by molybdenum etc.
Moly doesn't work on flat tappet cams as well as the Z & P.

For that reason, I use the older "diesel" spec'd motor oils.


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RE: newish Briggs backfires and won't start

Thanks again! I tore into the old engine (B&S Model 28R707 Type 1120-E1). It had quite a bit of damage from the broken rod, including a broken gear on the camshaft, and a mangled and disconnected oil slinger, but it gave me a better idea of the layout. It's an older model than the one I'll be working on, but apparently not too different.

As far as I can tell, here are the steps to remove the camshaft:
1. remove oil pan
2. remove oil slinger
3. lift out camshaft, noting orientation of the lobes
4. remove tappets (I assume; is there a reason to replace them?)

Then installing a new one should be just the same steps in reverse, right? How do I make sure that I put the new camshaft in at exactly the right rotation? More importantly, what am I missing? :)
Lewis

This post was edited by LRM2 on Sat, Sep 21, 13 at 12:22


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RE: newish Briggs backfires and won't start

Hi again,
I got a new camshaft in the mail today, pulled the engine, and removed the old camshaft. I cannot see a thing wrong with it! Except for a light sheen of clean oil on the old one (there were only a dozen hours on this engine), it looks exactly like the new one. I expected to see some obvious problem with the old one: a broken-off lobe, sheared shaft, something major. Am I missing something?
Thanks,
Lewis


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RE: newish Briggs backfires and won't start

Lewis: Might i suggest this: Go buy a new short block! I can tell, by your questions, that for you to try to rebuild your engine--will be a waste of your time, and money!
Rebuilding a lawn mower/tractor engine, or any kind of gasoline or diesel engine is way over your head in the "learned" category!
most of us here, who give advice to others, figure the "asker" has at least some idea of how to rebuild an engine, and other parts. I have been doing this work for some time. I started out as an auto mechanic, and then intto auto body repair and refinishing, and finally lawn equipment repairing. And, i still don't get into a job where i have to ask questions, without some in-grown knowlege of the job at hand.
you could ostensibly get the new cam in the engine, and other parts installed, too, and wipe it all out with one revolution of the crankshaft! (Ever hear of timing marks?)
I am now 88 years of age, and still learning about lawn equipment and how to fix the stuff.
A suggestion: Buy a short block. or: Buy a complete new engine. Or: Buy a new tractor. From what i have read, anything less will be a waste of your money. Then leave the rebuilding to folks who know what a cam-shaft is and how to install it correctly! JMHO: Rusty Jones


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RE: newish Briggs backfires and won't start

Rusty,
Thanks for taking the time to share your opinion. But this is actually a new engine - just put it in this summer. I don't plan to give up on it this easily. Having taken apart a similar engine and studied the repair manual, I am starting to get more familiar with it. I now understand what the camshaft is, what its function is, and even how to line up the timing marks ;)
I was just wondering whether the folks who suggested a new camshaft would have expected to see something obviously wrong with the old one.
Lewis


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RE: newish Briggs backfires and won't start

Look for one of the cam lobes to be noticeably "rounder" than the other one.
The one that didn't cause the push rod to move would only fail to do that if the eccentric was worn down.


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RE: newish Briggs backfires and won't start

Going back to the opening post, your statement about how the bottom push rod does not move in and out when you turn the engine...........was the indicator for a wiped out camshaft.
But is it possible that the push rod was simply not seated in its tappet? And you failed to notice that?
Are both push rods the exact same length and straight (not bent)?
With the push rods out of the engine, do both of your valves appear to be protruding the same distance and do both valve springs appear to be equal length?


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RE: newish Briggs backfires and won't start

Bill,
I measured the bottom lobe on the old camshaft and the new with a dial caliper. The old is slightly shorter then the new, but only .003" I don't think that's enough to have kept it from moving the valve at all.
Mownie,
I think you must be right about the push rod not being seated. I can't think of anything else. The rods are the same length and straight (altho one is aluminum and one is steel). The valves appear to be in the exact same position when the piston is top dead center.
I'll button it up and see what happens.
Lewis


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RE: newish Briggs backfires and won't start

Walt Conner recommends reversing the direction of the aluminum push rod.
To quote-
"I recommend removing and reversing the Intake Valve Push Rod when adjusting the valves.
If you remove the Aluminum Intake Valve Push Rod, you will see a worn area down a little ways from the Rocker Arm end. This is from riding on a fiber support rub block and the aluminum push rod will eventually bend or break.
Both ends are the same and reversing will prolong the life of the
push rod."


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RE: newish Briggs backfires and won't start

Thanks Bill and Mownie! It is running great now. I am still not sure how this could have happened, but glad to have it back running.
One tip for inexperienced folks like me: when you jump start an engine without hooking it up to the mower wiring harness, make sure you have a plan to shut it down :0
Lewis


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RE: newish Briggs backfires and won't start

Well, I spoke too soon. It was running great when I first started it, but then quickly ran into the same problem I started with. Valves were moving in and out this time, but it ran rough or not at all. I readjusted the valves a couple of times, and it would run a bit. I chased after this all day, until I finally figured out that the stud that the lower rocker arm pivots on was loose -- really loose! I think that may be the root cause of all of this. It would explain how the push rod could have come loose from the tappet, which had me baffled. I tightened it down, adjusted the valve yet again, and now it seems to be running OK.

But before I got to this point, I manged to shear off the idle adjust screw on the carb, flush with the body of the carb. It had a plastic end that kept the adjustment within a narrow range. The needle is still in there, probably adjusted to max open within that range, but I can't figure out a good way to remove it so I can replace it. Seems to idle OK, tho. As long as it idles OK, can I safely ignore it? And hope that it will vibrate out on its own so I can replace it?
Thanks again!
Lewis


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RE: newish Briggs backfires and won't start

As long as it idles OK, not really a problem.
How long do you think an engine should be run at idle?
Seriously, engines are meant to be run at wide open throttle when working and then idled not more than a minute or two if one wishes to idle the engine just before shut down.


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