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MG MGB Technical - Engine Lose of Power When Warm

I have a 77 MGB that I rebuilt the engine on. Problem is as the engine warms up I lose power. The warmer the engine gets the more power I lose. When first started the engine runs fine even with a load. Once it begins to get warm any load placed on the engine results in more lose of power. Idle is good whether warm or cold. Yes it is properly timed, yes the plugs are properly gaped, yes they look normal, compression is good, fuel pump is working properly, I have new fuel in tank. will lose power with or without the lights on, no the brakes are not hanging up.
I have no idea what to look at next. Help will be greatly appreciated.

Safety fast! (assuming the car will run)
Jim McDowell


what type of engine oil do you use on your engine?

Have you checked the fuel presure and is there everything o.k. with the carbs?


Has the engine been this way ever since it was rebuilt? If so, then it almost has to be a too tight clearance somewhere - either ring end gaps or piston-to-bore clearances or valve guide clearances.

Wayne Pearson

May have solved the problem! It seems the mixture was to lean in the carberator. Used the BLT 2010 and adjusted the mixture by increasing the CO. Car ran fine during the test drive, about 10 miles. Will run car during the week for at least 50-75 miles to see if the problem is really solved. Will post final results.

Thanks for the ideas!

Safety Fast!


This thread was discussed between 14/04/2008 and 15/04/2008

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