Re: Worrisome compression test results on my MGB 1
Tony, called me today and kinda filled me in your problem somewhat. A coolant pressure tester, a hand held pump device that attaches to your radiator cap will tell you if you have a blown head gasket or any other kind of water/coolant leak, You could probably go to a shop leave a deposit and borrow one, if you don't have, sometimes parts store even have a rent-a-tool program .
A short description of how to use a leak down test, first off I'll describe a leak down tester, It has fitting and hose simular to a compression tester that screws into the spark plug hole. Then it has a small aluminum block, that allows your air compressor line to come into it, two pressure guages, one reading the incoming pressure with a dial regualtor allowing you control how much air pressure you allow for the test (normally 100 psi), the second gauge will read the pressure being held into the cylinder. So with the cylinder at TDC compression stroke, this can be confirmed by removing the valve cover and looking a the valvetrain, you set the regulator on 100 pounds, then look at the other cylinder and whatever it reads tells you how efficent your engine is, say the gauge for the cylinder read 90, while the incoming air supply gauge is regulated at 100, this is telling you that you 90& efficentcy or 10% leakage. To further determine where the leakage is, you need to make access to the carb throats , exhaust pipe and crankcase ventilation pipe or hose, a air hissing sound from the crankcase ventilation tube/hose would be ring leakage, exhaust pipe would be exhaust valve leakage, carb throats would be intake valve leakage.
Now what beats people up about a leak down test is getting on TDC compression stroke, if you are TDC exhaust stroke, the exhaust valve will be beginning to open and you will get 100% leakage, if you are not exactly TDC compression stroke the air pressure of the tester will revolve the engine slighty and get it off TDC, so at TDC compression only will it give you a reading all other will result in 100% leakage, so if you get a reading other than 100% leakage, then you got a correct reading, period. make sense?
Now to give you an idea of what good/bad results in a leak down test reading, first off 0% leakage is really hard to achieve, I achieved it a handful of times, but not many.
In the race engines, or street engines no matter how much performance enhancments you have done to your engine, if you're not achieving good seal, then you're losing horsepower. In a race engine we look for 5% leakage or less, we may do a leak down test on a race engine and determine for example #3 cylinder's exhaust valve has 10% leakage but all other valves and rings are sealing good and producing good results. We might pull the cylinder head and lap just the one offending valve, because the leak down test has told where our leakage is, there no reason to address anything that is properly doing it's job, just what's not. On a higher milegae street engine, that's running fine, don't be shocked to see for example 20-25% leakage, if shows up to be vlave and the rings are not producing much of a hissing sound, then you might get a good vavle job, and boom, you got 20+% of your given horsepower back. Numbers as high as 10-12% on a street engine would be considered good, anything any better, well then wonderful. This is where a quality valve job comes into play, and alot of the time you won't get this at your corner machine shop, you just get a quickie valve job, for under $100. A really good head prep, complete cleaning of the head, decking it, multi angle valve job, checking stem hiegts, seat pressures, will cost you a good bid more than that, but you get what you pay for, all valve jobs are not created equal, and really a very important part of your end results with your engine. Replace any valve, guides, springs, stem seals, seats all will add to your final cost. Bottom line a good cylinder head prep is one of the very best things you can do to insure you getting the best performance from your motor.
I feel based on what Tony said about your head gasket leakage, then that might be the problem, but anytime you have leaking head gasket on a MGB, you have to make sure you don't have a crack, MGB heads crack if you look at them wrong, think about your time in the car, did you run it hot at anytime?
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