Compression test?

[Darter291]

I have a '73 914 w/2.0, D-Jetronic EFI that will not run for crap. I have swapped MPS, coil and other parts with a running 2.0 engine, but that did not fix. I also drained and refilled gas.

Could problem be a burned valve? Can you diagnose a burned valve (or eliminate that possibility) with a standard compression test? Compressions on all cylinders are 90 to 105 psi. Does this rule out burned valve?

Read earlier that need to run a "leak down" compression test. What is that, how do you run it, and what will it tell me? Any source of info on leak-down test equip/process?

Thanks!

[Dave at Pelican Parts]

A leak-down test is usually performed after a compression test that has shown lower numbers than you would like to see.

An air hose is screwed into the spark plug hole, and the engine is brought to TDC for that cylinder so both valves are closed. Compressed air is pushed into the cylinder through the hose, and either the rate or the pressure of the air going in is measured.

The primary benefit is that you can listen for where the air is escaping. If the hissing noise is at the intake, you have an intake valve leak. If at the tailpipe, you have an exhaust valve leak. If at the oil filler, you have leaking rings.

--DD