Well, let's stir things up a bit. When working on a 4 stroke, in most cases the manufacturer will give a spec for what compression should be, and if it is low, a leak down tester can be used to verify where the leak is, say a valve or head gasket, or rings. When working on a multi-cylinder engine, you should not see more than 5 or 10 % variance from one cylinder to the rest when checking compression, regardless of the actual number.
So why don't the 2 stroke folks publish compression specs? I talked to a STIHL engineer, and was told that compression readings don't necessarily have a lot of value, and that is why they don't advocate doing compression tests. I have seen an engine that had good compression when the rope was pulled through, and showed well over a 100PSI on a gauge, but the engine wouldn't run. I pulled the jug and the intake side if the piston skirt was extremely worn down from water scoring, and actually had a small chunk missing at the bottom, so this engine couldn't build any primary compression in the crankcase, and therefore there was no flow when the transfer ports opened, and the engine would not run. So that is one example of where the compression test was of no value. Depending on what part of the piston is damaged, you could have good compression but no start. I am not saying don't do it, but just be aware that good compression may not mean everything is OK in the engine. For small saws and trimmers, they should hold their own weight by the starter handle, as a rough quick check, but beyond that pulling the muffler and maybe the carb on a piston ported engine and looking in with a light tells me more than using a guage does.
Also, gauges vary alot. I had the chance to test 6 KD compression gauges at once and on the #1 cylinder of my truck they all gave a different reading by as much as 10 pounds, and I figured these were decent quality.
All I work on is STIHL, so does Husky or Dolmar or anyone else actually publish compression specs?
