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Cylinder not firing

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salmonkiller01

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"I have 1989 omc cobra 5.8l.At

"I have 1989 omc cobra 5.8l.At an idle cylinder #4 does not fire.
The carb, distributer , plugs ... all fine.
No abnormal noises comming from the engine , sounds fine.
Pulled the plug grayish brown running perfect.If i pull the plug lead off any cylinder at an idle the engine lopes down, but if i pull the lead off #4 no change.
At higher rpm it runs smooth.
Tested compression cold and it was at 120, other cyliders were with 130-145
Could i have a worn valve seat????
please any help will be appreciated"
 
Don't want to assume anyth

Don't want to assume anything so....when you pulled the wire off # 4 could you hear the spark jumping to the plug (wire is OK?) also have you moved/replaced the plug? When you did the compression test did you do it again after dumping a bit of oil in the cylinder to see if the #'s come up (rings?)
 
"Take off the valvecover and t

"Take off the valvecover and turn #4 to tdc compression stroke. Loosen the nuts for valve adjustment and check the height of the valve stems. If same hight, readjust. If Intake is higher than exhaust, probably a 'mushroom' valve. Head must down and valve changed."
 
"thanks for the info i will ch

"thanks for the info i will check out the valve heights tomorrow.
i made sure that the plug was firing, but did not add oil to the cylinder. i will also check it out and post the results."
 
"i looked into the compression

"i looked into the compression and added oil and cylinder pressure still low.The pressure was there an the first stroke and never increased, which indicated valves not rings.
Pulled the head and found #4 exhaust valve oblong shaped. Kinda what i thought but hoped i did not need to tear it down.
But ,on the other hand i found out that my motor must have less hours than the hour meter reads.1050 on the meter but when i pulled the valve cover it looked like new inside, intake gasket looked new(and pulled off not baked on).
the engine never uses oil between changes and the oil stays relativaly clean.
Or the engine was well taken care of.
anyhow thanks for the help."
 
"The other side is probably ok

"The other side is probably ok, but it is up to You. That sort of dammage happends from time to time! However exhaust or inntake valve? Normally the intake gets that kind of dammage!"
 
It was the exhaust valve in #4

It was the exhaust valve in #4 and i kinda knew that it could be comming because this spring i had my exhaust start leaking and that was the cylinder that was getting moisture(omc log manifolds). I have since changed to new centre rise exhaust manifolds and risers.
 
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