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MG MGA - Compression Readings - Am I off the hook?

Finally I managed to buy a compression tester ($8 from Harbour Freight - cant beat that) and performed my first compression test.

As you remember I bought my car un-inspected with all the risk that it brings.
I was pleasantly surprised with the readings after some scary posts I have seen in the forum.

The readings are as follows;

#1; 155 PSI
#2; 155 PSI
#3; 152 PSI
#4; 155 PSI

I guess this is pretty good.
Although the history of my car is fully unknown to me, I suspect that the 20k miles on the counter may indicate the miles since the engine was rebuilt. Would be good practice, right?

Correct me if I am wrong, but I understand that unless something catastrophic occurs to the engine, I should not expect to have to rebuild the engine head for a while? Am I really of the hook, or should I check something else?

The engine runs fine and issues I can appreciate, like smoke, misfire, oil consumption, coolant in oil, etc.

What I cant explain is that the 2 front cylinders always seem to run richer (carbon deposits around the spark plugs) after doing a carb. tuning 'by the book'. I turned the front carb. up one flat up to see the what happens.
Gonzalo Ramos

Gonzalo, when you checked the compressions, did you have the throttles held wide open?

If not, you will get lower pressure readings than you should as the closed throttles will reduce the amount of air flowing through the carbs into the engine.

So you may get even better pressures if you try this.

A friend of mine rebuilt an engine because of a low pressure reading and found to his horror that there was no improvement after the rebuild was completed.
I asked him the same question about the throttles and he looked at me blankly for a second or two and then realised his oversight.
When he checked the compression pressures again with the throttles wide open he found much higher readings and he realised that he had rebuilt the engine for no reason at all.

The worse thing was that he was a mechanic by trade and the engine was for a customer!

He never did tell me if he came clean about this to his customer.


PS I seem to recall that 175psi was the kind of ballpark figure for compression pressure but I may be wrong.
Barneys site will have the correct figure somewhere.
Colyn Firth

Just checked on Barneys site Gonzalo and it looks as if your compression readings are ok, pretty much as they should be.
Colyn Firth

Thanks Colyn!

Indeed I did do the compression test with the throtle open, but I didn't notice much difference in doing it with the carbs in the idle position.
It takes less for the compression to reach the final figure with the throttle open, but the final figure is the same in both.
I guess the key is to crank it long enough for the pressure to stabilize.

My experience, anyway...
Gonzalo Ramos

This thread was discussed between 26/07/2010 and 27/07/2010

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