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MG MGA - Engine 'Blowby' & Smoking


1961 MGA roadster.

Rebuilt engine ( not by me ). Compression good ( over 150 cold, slightly lower hot ) but apparent UNCOMBUSTED oil at muffler !!! Not just carbonized but "wet ".

Great power., but smoking, especially on acceleration. Runs without smoking while cold. But will ALWAYS smoke on start up. Supposed rebuilt head with ARP studs.
Are there any definitive tests to diagnose this problem.

This is driving me crazy. Aside from a complete rebuild or replacement engine...any ideas ?? ARGGGHH.

Help .
DA MacFarlane

Pull the plugs and check them, at least one should be very oily. Since your compression is good it rules out things like blown gaskets and cracked heads unless the crack is into an intake or exhaust valve port. This is also way to much oil for leaky valve guides. May be that the expander was left out of one or more oil rings. I would head straight back to the rebuilder for a discussion on how he is going to solve it.
J Heisenfeldt

Are you running the standard MGA breathing engine breathing system?

Steve Gyles

I'll propose that you are running very rich in one or both carbs which is thinning your oil and as a result the oil is running through the system.

Does your Oil turn black in short order after you change it?
T McCarthy


I have the aluminium type rocker cover ( the one with the small hole in it ) I have heard and read good/bad things about these.
Also the oil does turn dark fairly quickly.

Would the "umbrella type " Felpro seals help ?
DA MacFarlane

No, I think you want to adjust your mixture and change your oil (and your filter); a rich mixture leads to gasoline washing down the sides of the cylinder wall and into the oil. This causes significant wear and oil seeping back over the rings.

Take a look at your plugs, are the electodes a nice even tan or are they black?

T McCarthy

If it is smoking mostly when hot and accelerating it suggests piston rings to me.

Has this been happening ever since the engine was rebuilt? Is it getting worse or better? How many miles have you put on the rebuilt engine?

It might be a problem with the rings bedding in. You need to do a pressure leakage test of the cylinders.

Malcolm Asquith

I'll check the plugs tonight.

It smoked slightly when I had it done but has been getting worse.

I will lean doen the mixture as well.

DA MacFarlane

DM, don't lean it down unless its really running rich.

Additionally one way of testing my idea is to see if the oil in the muffler reduces when the oil is new and therefore thicker.
T McCarthy

This thread was discussed between 17/06/2008 and 18/06/2008

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