I have an older OS 48 4 stroke that I picked up from a club auction about a year ago. I've rebuilt the engine and replaced the bearings and the carb (newer OS 55 carb).

Today I was running it on the test stand and it seemed to run fine except for an inconsistancy in high RPMs. I got the high speed needle set at about 10K, backed off to about 9700 and then let it run for awhile checking it periodically with the tach because it just didn't sound right. It sounded like it would lean out and then slow back down ... It would idle fine. The RPM's varied between 10K to as low as 9400 RPM.

It did the same thing with the old carb, which is one reason I decided to replace it (that and the fact that I wasn't getting much more then about 8800 RPM and the needle valve was shot).

So, what can cause an inconsistant high RPM? Bad valve spring? Valves out of adjustment? Air leak? Any idea where I should start looking? Or is this normal ... I'm fairly new to the hobby (about two years) and even newer to 4 strokes.

Thanks in advance for any help.