Can't figure out missfire

T

tkos115

Well-Known Member
So.. I'm trying to figure out an issue on a twin cylinder 18hp tecumseh engine. When it's running it seems to run a bit rich and it pops and backfires a bit, mostly at idle speeds. But it will still do it at all speeds. If I take off the plug wire on the left head it almost makes no difference in the way it runs. But it does stop poping. If I pull the other side it will still run but a huge difference in how it runs but it will still run. It will also stop popping too. Spark to the left head is really good, nice blueish white. Compression when cranking is around 70psi. The right side is actually less at about 55psi. I have cleaned both carbs and no difference was made, I even replaced the left head carb but it stayed the same. When it's running I can spray carb clean into the left cylinder carb and no difference I'm run quality. If I do it to the other carb it will make a huge difference in how it runs. I've checked the valves and intake and everything looks good. No intake leaks and both rockers move fully and the same amount.

Any ideas? I'm totally stumped..

Engine numbers:

OV691EA-600824C​

 
your problem is always on the side with the smallest change in RPM and sound on a cylinder balance test. The reason is that the engine is relying mostly on the strong cylinder to run.
 
Take both the kill wires off the coils
If the problem goes away you need to replace ( or fit ) diodes to the kill wires .
The kill wires can pick up FR from the opposite plug causing the coil to fire at the wrong time or fire twice .
 
Ok so... took the kill wires off both coils the lh head is still the weak cylinder, and almost no change in engine running. I swapped the ignition coils and it didnt change anything either..Could the cylinder head be bad somehow?

Cranking psi on LH head (weak cylinder) 90psi
Cranking psi on RH head: 70psi.
 
I'll try to pull the head today and take a took. The only thing throwing me off a bit is that I can see both rockers on that side move the same amount down on the other cylinder does. It also builds more compression when cranking over too.
 
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