Low compression? Here's a way to find out what's wrong.
First thing you will need is a spark plug adapter for your air compressor. You might be able to find one at an auto parts store but most of the desk jockies there will look at you as if you had lobsters coming out of your ears. So build one yourself. Take an old spark plug that came from the car and beat it with a hammer till the porcelain is crumbled and can be removed. Careful not to damage the threads. Now get an air chuck fitting that will fit your air system. Screw it into the end of the plug where the porcelain was and braze it in place - it has to be a good airtight seal. You can epoxy it in place if you don't have a torch.
Now screw it into the plug hole of the cylinder that has low compression. Hand crank the engine over until you feel air pushing out of the fitting. That will mean that you are coming up on the compression stroke and both valves will be closed. Now lock the crankshaft in place using a socket on a breaker bar placed on the crankshaft damper pulley. Wedge the handle tightly into the frame somewhere - make sure it is locked in place in tightly else it will spin around during the next step and whack you in the head!!
Now put your air hose onto the fitting and slowly begin to increase the pressure on the cylinder.
As soon as you hear air leaking somewhere stop increasing the flow. Use a length of heater hose to listen to the hole where you add oil. This is normally on top of one of the valve covers. If you hear air leaking there then you have either badly leaking rings or a cracked or hole in the piston.
Listen at the tailpipe. If you hear hissing in there then you have a leaking exhaust valve.
Listen at the throttle body intake. If you hear hissing there then you have a leaking intake valve.
If you hear hissing at all three places they sell the car as fast as you can!!
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