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MG MGB Technical - Low Compression, Black Clouds :)

I got a '72 B that was treated to a rebuilt engine 8-12k miles ago. This was about 8 years ago, I have no recourse so I have to fix this engine myself.

Anyway, the new engine started to blow serious serious serious black smoke. I did a compression check and found 110+ in the first three, and around 80 in the last cylinder. I took off the carbs in preparation to take the heads off, and found the carb closest to that 80 lb cylinder to have a puddle of engine oil in the intake and in the carb throat.

Before I take the head off, is perhaps a valve not closing all the way?

The valves look ok, but perhaps one is chipped?

Or am I goign to likely find a broken ring?

---John
John Elwood

The only way I can see engine oil at that point is if it's coming in from the breather John. That might be a breather valve fault, so check and clean, but could also be caused by excess piston blow-bye causing excess crankcase breathing into the manifold. This would agree with those compressions which need investigating anyway. So, I'd continue to take off the head and check the valves but, as said, excess beathing is usually piston blow-bye and you could have broken rings or worn bores. If the bores and pistons are OK you can take the sump off and re-ring it without taking it out. Rich.
Rich

John. As Rich indicates, the low cylinder, at 80 psi, may indicate a problem. With the minimal amount of miles you indicate, the piston rings may have seized to the pistons. Rich may well have a valid point. You may need to pop the head off and re-ring the pistons. However, before you do this, may I suggest that you pour some Marvel's Mystery Oil into the bores and let it set for a while? Your engine should be a low compression engine showing about 140-150 PSI at its best. If, however, the rings have stuck to the pistons due to lack of use, the figures are significantly reduced. This is what I believe has happened in your instance.

I would put a battery charger on the battery. Pull the spark plugs (mark the leades if you are not familiar with the firing order of an MG) and pour about a quarter can of the Marvel's into each cylinder. Let it set for several days--a week is not too long. Then, turn the motor over on the starter, with the plug out, for several seconds to vent the oil out of the cylinders. Hopefully, the Marvel's Oil will free up the piston rings.

Install the spark plugs, reattach the plug leads and drive the car for a while. Then, perform another compression check, making sure the engine is warm and the throttle is fully open. If the compession check reveals the compression has come up, you do not have to worry (if all are within 10% or 15 psi of each other--opinons vary). If the compression readings do not come up, follow Rich's advice and re-ring the pistons. Les
Les Bengtson

Yes, certainly worth a try, gummed rings are possible if it's not been run a lot. I don't know Marvel but we have Redex, a thin red oil designed for this. Use it exactly as Les says, pouring into a hot engine helps, and leave a day or two. A sticking valve might account for uneven compressions. If you are really lucky dribbling Redex or one of the other valve cleaners through the carb whilst running may clear it. Rich.
Rich

good advice - and for everyone thinking of pulling sleeping Beauty out of the land of Nod - the use of an upper cylinder lubricant for the first (and last) tank of the year will prevent these probs.

My money is on a gummed up valve (s)

Roger
RMW

Just a question, have you checked the valve
clearances?
Ralph
Ralph

This thread was discussed between 19/03/2005 and 22/03/2005

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