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MG MGB Technical - Low compress and oil consumption

I know this is a minimum of a cylinder head problem but just incase its worse I thought I'd share it with you guys.

'76 MGB GT unmodified running it on 95 octane unleaded + additive (octane+lead substitute)
Its always been a little slow, though never driven another B to compare it with.

Also only do ~2000 miles per year max and the last year noticed a huge increase in oil consumption. No visible signs of oil leaks or smoking exhaust. Or oil in water.

Anyway decided a few weekends ago to have a good poke around.

All Plugs came out with according to the Haynes Manual "Heavy Deposits" if you have the manual to hand page 33, the picture is pretty much exactly what they where like with No.2 being the worst!

So compression test results
Note the engine was only warm
No.1 160
No.2 85
No.3 155
No.4 150

Retest on No.2 = 92
Retest on No.2 + teaspoon of oil = 100
Retest on No.2 = 95

Ok so definitely something wrong with No.2 but not sure whether its compression escaping past piston rings or past the valves.

Before I pull the head off, I'd like to do some more investigation. What do you think I should do next?
T I Lyons

Had a similar problem when I first bought my B.

The fact that compression doesn't change much
with a teaspoon of oil suggests more of a
head/gasket/valve problem than rings.
I would suggest pulling the head and sending
it to a machine shop to check for cracks,
followed by a valve job if the head is
ok. The exhaust valve on
No. 2 could be burned, and/or the head gasket
leaking.

High oil consumption can occur due to leaking
oil control rings, even if compression is ok.
If the plug fouling isn't due to worn valve
guides, then rings must be the problem.
Ronald

No point in doing any more investigation!

Pull the head now and strip it down - you will need to decoke it if you have heavy deposits on the plugs.

Once off you can inspect the valves and see if one or more are burnt.

You can also examine the tops of the pistons for chunks missing and, by sliding a 2 thou feeler gauge down the side of the piston you can check for stuck rings on number 2.

At that point you can decide whether to have the head converted to unleaded or whether to go for a fully reconditioned engine - probably the cheapest option in the long run.
Chris at Octarine Services

Thanks guys, needed the second and third opinion.

The car came from a relative will check out some history, ie rebuild/reconed..

Looking like a full recon-ed engine

cheers
Trev
T I Lyons

Personally, I'd look into doing a bottom end "freshening" even if the oil pressure was good. I like to KNOW what is down there! If the bores are in good condition then a light honing and re-ring is nearly free when compared to the frustration of an engine that never seems to run right.
I suspect you will find that the rings never did seat properly when you get it open.

Mike!
mike!

This thread was discussed between 25/07/2004 and 26/07/2004

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