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MG Car Repair/MGB compression \ damage?


hi Howard - my 74 MGB was running fine recently, but I experienced some flooding issues and gas got into the oil at some point, not sure how long I was driving it like that. Since then I have flushed out but now cannot start it. Sadly I checked compression and found all cyl were only 75 - but just prior to gas incident all were 140. I did change out the carbs and removed and reinstalled the inlet manifold and did not renew gasket. Do you think I have damaged internal bearings or possibly have a leak elsewhere?
Appreciate any insights of where I go to now.
regards P

Hi Paul,

That's not a good sign but don't consider that the end yet. Run the compression test again but be sure to open the throttles wide open during the tests. Then put about a table spoon of engine oil in each plug hole and run the test again (called a "Wet" test). If the test is ok with the throttles open don't worry about it. If it is still low note the second "Wet" test. A Wet test should be higher then a normal "Dry" test but not more then 10% to 15% higher. If it is more then that then the rings may be damaged but still not for sure.

However, there is one more thing you need to do. If you got a low compression test even with the throttles open and got good compression with the "Wet" test, try to start the engine just after the "Wet" test and if it starts note the oil pressure. This is a clear indication of any bearing damage. With fresh oil you should see 25 PSI or higher at idle and 45 PSI to 70 PSI at 2000 RPM. keep in mide it takes a little running to get the thin oil out of all the oil galleys.

The reason you need to run the engine a little after the "Wet" test is because just turning the engine with the starter motor after removing the thinned out oil, will not get fresh thick oil into the rings because the rings are lubed by the oil that is slung out of the rod bearings and that takes a little running to get the fresh oil into the rings. It also takes a little running to get all of the thinned out oil out of the oil galleys so it would be wise if it is ok to change the oil again after just a few miles of running to get all of the thinned oil out.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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