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MG MGB Technical - Wierd Compression Readings

Can anyone out there please help me decide what to do about these compression readings that I got...#1=150,#3=145,#4=50,#2=0. What could it be the valves,piston rings,or any other things it might be. Also what other test could I do. I am about ready to pull the engine and do a complete,but I would like to know if I really need to. Have been working on this 72 b-gt for about a year and have had this car for about 30+ years and I really miss driving it. Thanks...
RP Padilla

Has the car been running? how does it feel? Is it using oil, ( blue smoke)? But if one of the cylinders really registers 0, perhaps the rings have got stuck? Try the trick of squirting a little oil directly into the bore via the plug hole then measure again. What is the mileage? It may turn out that it might just need a 'ring job.' I would not expect some bores to be very good on compression then other right out? They would all have been subject to the same work, more or less. What's every body else think? Mike
J.M. Doust

RP

John Twist did a good piece on this subject, it worked perfectly for me, I made an adaptor from an old sparkplug and an air line fitting welded together. John has been away for a while but is now back and I'm looking forward to some more very instructive videos.

Click on the link below

http://www.youtube.com/watch?v=hD02_UZ_ZuU

Bob
R.A Davis

It may be stuck piston rings after standing for a long time. worth trying a wet test with a little oil in the cylinders. If so a soak in penetrating oil or your = to redex may get you out of jail.
Stan Best

This may well be sticking valves, especially if the engine hasn't been turned over for a long period of time. RAY
rjm RAY

Thanks I will try the oil or penetrating oil. How long should I let soak? The car did not run for 13 years and after re doing the SU carbs (new floats and gaskets) it started right up and ran. It was overhauled @ 49,000 original miles and now has 94,000 miles on it. I am not giving up as I really enjoy this car. No real blue smoke but some was coming out of valve cover breather. On the sticking valves what do you recommend other than pulling head and redoing it? I am ready to try anything to get on the road. I really don't have a problem with doing a complete which I might just do. Will post pics later of car.
RP Padilla

The wet compression test and soaking the rings a 2 different actions. 1) put "some" oil in the offending cylinders, engine oil is good. Enough to give a seal all round the cylinder but not so much as to significantly reduce the combustion volume. I used 2 squirts of my oil gun when I did it. Now do your test, if you get good numbers then it is probably piston ring trouble. If it is then you may be able to free them with nothing more than a soak in pentrating oil, If 2 or 3 days doesnt work then its engine out time. If this makes no difference then yes, stuck or burnt valves are a possibilty. The first can be seen with the rocker cover off, otherwise its time for the head to come off. Fairly low tech, but rather heavy to lift on these cars :-)
Stan Best

If you have access to compressed air, checking out the valves is an easy task. Just thread an adapter into a spark plug hole, making sure that the engine is at TDC on the compression stroke. This can be confirmed by removing the rocker cover and making sure that both the inlet and exhaust valves have clearance between their respective rocker arms. Apply compressed air to the cylinder and listen to the carburetor inlet and then the the exhaust pipe. Repeat on each cylinder. If you hear any air escaping from the carburetors or the exhaust system, you have a valve problem. If you have air coming from the crankcase, you have a piston/ring problem. RAY
rjm RAY

Thanks RAY, J.M. Doust, R.A. Davis, and Stan Best for help you gave me. See Head Gasket it was thread and would appreciate a reply from you.
RP Padilla

I can't see stuck rings causing a zero, there is a lot more surface area covered by the piston still going up and down than by the rings. Get the rocker cover off and see if the valves are going up and down. If it's a *burnt* valve you should hear chuffing in the exhaust or the intake, depending on what valves are burnt. Without that it could be a holed piston. I'm surprised it even ran with one at 50 and another at zero, did you go back and check 1 and 2 were still at 150 and 145? Could be the gauge gave up (like mine did, perished hose between plug hole fitting and gauge).
PaulH Solihull

Don't you just hate it when someone starts a new thread to give the answer to a question they had themselves raised ...
PaulH Solihull

This thread was discussed between 06/03/2011 and 11/03/2011

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