QuestionQUESTION: Subject engine is in a John Deere LA125 2008. Over night, engine changed in sound. Sounded much louder with more of a flopping sound and lots of back fire. Checked the exhaust for burst baffle: it's ok. Checked fuel pump: ok. Checked compression on the compression stroke: ok with very little leak over time. Pulled the valve cover and checked for valve timing. I seem to have some air coming out of the carb in the early part of the compression stroke. The intake valve seems to close too late and in 2 steps (95% closed at the bottom of the intake stroke with last 5% in the first 20 degrees of the compression stroke. You can see the rocker move in two steps so it doesn't look like a sticky valve. Is this normal?). Have you ever heard of a camshaft becoming misaligned all of a sudden? Unless the gear is plastic and broke a tooth, how can it skip? Any other idea? Can I use carb/intake cleaner to free a possible sticky valve or will it destroy the carburator?
Thanks, Vince
ANSWER: There are plastic teath on some camshafts these days but usually more than one teeth break.
The valve action you observed is probalby caused by the compression release. This is normal and is usually ground on the exhaust lobe or could be spring loaded on some cams.
I would suspect the back firing is caused by a lean fuel mix. Have you removed, dis-assembled and cleaned the carb with compressed air from an air compressor?
If you remove the air filter with the engine running does the problem get worse? If you hold you hand over the carb, choking off the air, does the engine run rougher or smoother?
Can you please send me the engine model type and code number so I can look up the parts lists?
Eric
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QUESTION: Thanks Eric.
The engine is Model 331877 type 0113B1 Code 080519ZD
I took a good look at it and some measurements. The exhaust valve works fine and seems in good timing. The intake valve seems to be my problem here. It opens at the right time during the intake stroke and starts to close just before it reaches the bottom but it doesn't close completely until it is 1/2 inch from the top of the compression stroke. Could your decompression thingy be on the intake lobe and stuck in the "open" position. Is it activated by or deactivated (should I say)by centrifugal force? This gives me a lot of blow back in the carb as I said yesterday and has changed the sound of the engine drastically. If it is the compression release, can it be unstuck without opening the engine. I don't see it on the illustrated parts breakdown. As for your other questions, the engine barely runs. Too much blow back and backfire to tell. If it would be too lean, I would get back fire but the engine wouldn't sound like a compressor.
Thanks for your help. Vince
AnswerWhat is the valve clearance? Link below covers valves.
http://www.briggsandstratton.com/engines/support/frequently-asked-questions/Serv
Compression link:
http://www.briggsandstratton.com/engines/support/frequently-asked-questions/Comp
An inexpensive, and accurate method to check the push rods is to roll them on a piece of glass. A straight push rod should roll flat and smooth.
Eric
