[Bill Hall]

Compression vs leak

A Compression Test is merely an indicater of overall integrity. The correct test would be a Leak Down test. Motors with 12 or 15% leakdown can still reflect a fairly good compression reading and still have valve issues. Most small engines will limp along inspite of poor compression or bad leakdown. So your probably looking at ignition timing if you've verified that the valves are opening and closing correctly. There is however an outside chance that this motor was asembled on a friday and the cam gear was assembled a notch or two off! Verify that your on TDC and both valves are closed. Seen it before.

 

[Bill Hall]

Compression vs leak

A Compression Test is merely an indicator. The correct test would be a Leak Down test. Motors with 12 or 15% leakdown can still reflect a fairly good compression reading and still have valve and or other issues. Most small engines will limp along inspite of poor compression or bad leakdown. A leak down tester can be had at any decent auto parts house and requires an air compressor to work. Kinda spendy but a good investment if your gonna play with motors. The fancy name is "cylinder pressure differencial" . In other words If you put a 100 psi in and the guage reads 90 psi you've got 10% leakdown. It's the difference between how much you put in and how much leaks off. Then you gotta find it: listen for the hiss of air in the intake or exhaust for valves, listen at the oil filler for rings or piston/cylinder damage, and finally listen around the headgasket. Some motors leak in more than one place! The test must be made @TDC compression stroke. Myself I woudnt go to that extreme yet. Your time and money would be better spent rechecking the obvious. More than likely your probably looking at ignition timing if you've verified that the valves are opening and closing correctly. There is however a good chance that this motor was assembled on a friday and the cam gear was assembled a tooth off! Seen it before. Still makes OK compression. A buck twenty five seems a little low for a new one lunger! Any one agree?