Briggs 18hp Opposed Running Problem

12 years ago

My 18 hp Opposed Twin is not running well at all under load. Idles ok, half throttle ok, full throttle a bit rough but manageable; all until the mower or gear are engaged. Once the PTO is engaged, for example, the rpm drops to half and sometimes dies. Same if I'm going up a hill. In short, no power. I do manage to keep it going, it will misfire and shoot flames out the exhaust. Cannot understand the governor mechanism but it seems to be free to move about so I'm guessing it's ok. New plugs, fresh gas. I'm thinking ignition as it's low on power and misfiring = timing? Timing appears to be set via flywheel + magneto so not adjustable? Hope it's not valves. Help.

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Comments (8)

walt2002
12 years ago
I would make sure that it is running on BOTH cylinders. My guess is that it is not.

Flywheel key MAY be seared or partially. You can check this by removing the cooling shroud, flywheel screen, flywheel nut and looking down the keyway to see that both the crankshaft and flywheel keyways line up.

Don't forget to tighten flywheel nut to 150 ft. lbs.

"it will misfire and shoot flames out the exhaust."

Could be loose Valve Seat.

Walt Conner
menelson71
Original Author
12 years ago
Hello Walt. Thanks for your advice. Do you happen to know the size of that flywheel nut? Don't know if I have a socket or wrench big enough to get it off. Also, do I have to remove the other two screws that appear to hold the fan to the flywheel? They're all rusty so might be tough to get off.

I think it is running on both cylinders as I have disconnected the plug wire on the left side and there is a noticeable difference in running conditions. Haven't repeated on the right but will do so soon.

Let's say the keyways line up, any other options? How to fix a valve seat? Exhaust or intake valve seat?
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menelson71
Original Author
12 years ago
Walt,
It is an exhaust valve seat that is completely loose. How to fix? Thank you
Mike
walt2002
12 years ago
Depends on how loose it is. IF the block is wallowed out, no practical fix. IF the valve seat is just a little loose, use center punch out a little ways at 3 approximately equal space points around it then fill in between with 1/8" or so flat punch peening the block metal over the edge of the valve seat. Lap valve to seat.

You really need to check your cooling fins as overheating from plugged cooling fins is a prime cause of this.

IF the block is wallowed out, only a good machine shop can do a good fix and cost is not practical.

Walt Conner
menelson71
Original Author
12 years ago
Ok, thank you. Tried a bit of JB weld and some mild punch peening. Ran good for about 30 minutes and then came apart again. Will try one more time with some more aggressive punches.

About the cooling fins, I got the tractor from a friend who had let mice fill all cooling fins and passages with "stuff" so not surprised I'm having this problem. The big surprise is that it is only on one side.

Good news is that it appears none of this has been done before so, with some luck, I just might get it to work. Thanks again.
walt2002
12 years ago
"Ok, thank you. Tried a bit of JB weld and some mild punch peening"

JB Weld is no good for anything like that. It turns to liquid at a relatively low temp. You have one, maybe two chances to fix this and you need to do it "like you mean it" or block will be damaged to the point only very expensive and time consuming profession machine shop will salvage it.

You need to clean the valve seat recess out good as well as the seat. Now you will need to lap the valve in after re-installing it.

Walt Conner
menelson71
Original Author
12 years ago
Thanks Walt. Will do. Also, please describe the valve lapping procedure so I can get that right too.
mownie
12 years ago
If a picture is worth 1,000 words.............a video is easily worth 1,000,000 words.
Link below will get you to a video showing one man's demonstration of valve lapping on a Briggs L-head engine.
Once you get to the web page there are numerous other vids on the subject for your eye if you want to get a better feel for this procedure.

Here is a link that might be useful: Briggs valve lap