I've got to agree with BLW on the likely culprit being the carb adjustment. It sounds like an over rich low end. You can't just adjust the low end though and be done with it, you need to go through both adjustments a couple times to get things right.

One other issue with inverted and semi inverted engines is that they collect a lot of oil in the rocker cover. When you first start them, it has to be heated up and go back into a mist state than can be circulated and burnt off. The richer the engine runs, the more oil will accumulate. I had a similar issue with an O.S 52 that was mounted in a Phoenix Fun Star. The design is that the engine is mounted a 7:30 looking at the prop. It smoked and sputtered and required a very long warm up then ran just great, never a dead stick. Idecided to check the valves at the field one day and when Itook the rocker cover off, there must have been a half tablespoon of oil in it. I thought of putting a drain nipple on the rocker, but I killed the plane and the problem went away for now.

You might try storing the plane with the engine upright and see how it behaves that next trip to the field.

Last point, comparing compression between makes is not very productive. I've a couple Magnum engines and they have almost no compression compared to a like size OS. The Saitos I have seem to have more than the O.S engines. They all feel different. They all run well.

Don

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