View Full Version : Engine: low commpression on one cylinder

jim payne
07-12-2011, 11:48 PM
My 50 Champion has 3300 miles on a complete rebuild. At 2000 miles we discovered that the compression in one cylinder was at least 25 lbs lower than the others. It came up when oil was put in cylinder. The rebuilder told me to wait until I had 4000 miles, thinking that possibly the rings did not seat properly on that cylinder. The rebuild was 6 years ago. I don't put many miles on the car. The engine runs quite smooth. any thoughts on what could be wrong?

07-12-2011, 11:57 PM
You've determined which cylinder is the low one.

With the wheels blocked, the transmission in neutral, and the key off, crank (by hand if you can) the engine around until the leaky one is at the bottom of the intake stroke. (pull the plug to see when its' at bottom) put your finger in the spark plug hole and turn the crank enough to feel pressure on your finger. Remove finger and replace spark plug, tight.

Remove the air filter and the oil fill cap. rotate the engine and listen for a pronounced hiss. If you hear it at the carb, you have a leaky intake valve, if you hear it at the exhaust pipe, you have a leaky exhaust valve. If you hear it at the oil fill tube, its the rings.

You might want to compare other cylinders for hiss, they all may do it some.

jim payne
07-13-2011, 09:22 AM
Thanks, I should have noted that the rebuilder determined the problem to be in the cylinder, no valve leak or blow-by. If its a seating problem it seems odd to me that it would only be on one cylinder. I have not had the commpression re-tested yet.

07-13-2011, 10:47 AM
Sounds like the builder might have broken a ring ? Just a guess, from the ten thousand I have worked on.

07-13-2011, 09:15 PM
I'm with Tex.......broke a ring on that cylinder, or forgot to install one of them <G>

07-14-2011, 12:58 AM
FWIW, I've seen builders not know how to properly re-install the wrist pin locking wedge bolt. If the bolt works loose, the wrist pin will move out and score the cylinder wall. The scored marks will leak compression.

jack vines

07-22-2011, 10:10 AM
I can say with confidence,if the builder did something like that ie broken rings or a wrist pin he does not need to be building motors for people.he needs to go back to the basics.I say this being a engine builder.those are rookie mistakes and are things that should have been done with care.sorry JMHO I look down upon builders that slap things together to get them gone.gives me a bad name as a builder.I am very sorry for your headache.....never should have happened period.and he knows that!