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MG MGB Technical - Compression Readings

Curious if compression has dropped, I tested my '72 MGB with nearly 108K miles yesterday, immediately after taking it out in 90 deg. heat (hot engine). Readings were #1, #2 & # 4 @ 145 PSI, #3 @ 135 PSI. When I originally got the car 9 years ago, the readings were all between 150-155 PSI with 96K miles on the engine. I have records going back 25 years, and to the best of what I can see the head has never been off. The car starts OK, and runs well after it is warmed up, I do get a slight valve popping sound at idle, when you listen to the exhaust standing in back of the car.. Oil consumption is minimal, at a quart or less/ driving season. Would the consensus be to leave it alone, and wait for it to decline further, or rebuild the head this winter? I don't want to spend the money to rebuild the entire engine if not absolutely necessary. Your opinions appreciated.
Joe Dufresne

I would be tempted to do a head off and lap the valves in again, not a big job or too expensive, unless you find something nasty like seat recession. Or, if it is running well, good idle and pick up, leave it alone. Sounds like you don't do that many miles.
Art Pearse

Joe.. I would not touch that engine until you have at least 15 LBS diffenence in compression.
Sandy
SANDY SANDERS

Joe. I agree with Sandy. When I purchased my GT, the compression readings were 110-120 for the four cylinders. Drove the car, with no engine associated problems, for nine years like that. I do not consider your compression readings to be a significant problem.

I am not sure what: "I do get a slight valve popping sound at idle" means, so I cannot address that issue.

Les
Les Bengtson

On the face of it times three, going by the compression figures there is nothing wrong with it worth doing anything. However popping in the exhaust could be a poor exhaust valve seal, but I'd expect a bigger difference in compression than that. You could try removing No.3 plug lead, then replacing it and removing No.2, and seeing how the popping changes, that should help indicate whether it is 3 or not.
Paul Hunt 2010

Leave it alone. Those numbers are not that bad at all. RAY
rjm RAY

A subsequent thought, is the popping a regular beat, or irregular? If the latter then it definitely isn't a valve but a light misfire due to some other reason.
P Hunt

The popping sound is a regular beat at idle. Also, when letting off the gas when descending down steep hills, I often hear popping. It has been suggeted to me that air is getting into the exhaust system causing that, but I am not so certain that is the case. I have tightened the 6 exhaust nuts as tight as I dare, but the studs are old and need to be replaced, another winter project.
Joe Dufresne

Popping on the overrun is usually a gulp valve fault. People do say it can be air getting into the exhaust as well. That's different to a regular beat, which is more likely to be a valve.
P Hunt

This thread was discussed between 12/07/2010 and 15/07/2010

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