Micropuller,

Measuring the chamber and calculating compression ratio is not the way you tune a naturally asperated 2 stroke cycle diesel. Tuning mus be done as a combination between mixture and compression ratio. Normally a diesel will burp when under-compressed. Screwing the contra-piston in will increase the CR and the burping will cease. Mixture then can be leaned until you begin to here a slow-down. Also remember that your initial starting compression setting that will make the engine run steadily will be too high for running at operating temperature so that without touching the compression you have the engine overcompressed. With my diesels I find the happy spot after it is up to operating temerature and very seldom change the compression after that. When I first start it, it will burp a bit then smooth out when up to operating temperature.

It is a common mistake to try to compensate for over rich condition by over compressing. So initially start rich, lean until is slows a bit. Back off the compression and see if it picks up. If it does you're headed in the right direction. The idea tuning is as lean as possible with as little compression as possible. I then richen a quarter turn or so for no other reason then it makes me feel better.

Randy