273 Compression Test Results

I've confirmed that I have a bad exhaust valve. If 120psi indicates healthy rings, then I guess I will get the heads redone, install a gasket set, paint it up nice and pretty and call it done.

Here's what I did, in case anyone can benefit from the experience. Feedback, tips and better techniques welcome.

I rigged up a fitting to put comressed air into the cylinder. I used a fitting that came with my compression gauge and an air hose nipple. Screwed it into the spark plug hole and chucked on the air hose at 110 psi. Lot's of air coming out the tail pipe. Bumped the starter, now lots of air out of the carb. Kept trying to get it to stop at a spot where both valves are closed, but no luck.

Cut the air supply, pulled the valve cover off and bumped the starter until both lifters were loose. Valves definetly should be closed now. Turned the air back on and still blowing out the tail pipe. Repeated this at least 5 times. WTF?

Those of you with more experience already know where this is headed. Myself being significantly less experienced had to do some head scratching. Quit for night, closed up the garage and within 10 minutes it dawned on me. Air pressure was driving the piston down until the exhaust valve opened. After some trial and error with the regulator, found that 30 PSI was not enough to crank the engine.

Set the valve cover back on and repeated the test. I could hear and feel a tiny bit of air coming out of the breather indicating some blow by, but the engine was cold and dry and some air is to be expected past the ring gaps right?

The real smoking gun came when I discovered that if I pressed my cheek against the tail pipe, pressure would build up. That's not supposed to happen if the exhaust valve is closed. Repeated the test on another cylinder and felt and heard similar air at the breather, but nothing at the tail pipe.

Mystery solved.