This is a cool diagnostic tool. With the pressure of 100 going in, we read 30 psi holding. Now with all the covers off and exhaust open, we could listen to the areas with a garden hose to pinpoint the problem area. Intakes sounded quite, exhaust valves sounded quiet but the crankcase was hissing like a snake. OK now we have a better understanding of what to look at.
Prior to this we read all the FAQ stuff on the F650 site and everything pointed to a counterweight shaft seal. But with Max's info on low compression and our investigation, we're now looking at that plus some ring/piston stuff. The way the Rotax 650 (I think its the 652 engine) oil system works (Dry Sump), the lower end does have some oil in it before scavanged back up to the tank. So any additional pressure will drive it out through the vent to the airbox. It's even weirder than that, I need another post to diagram that.