ORIGINAL: w8ye
The crankcase on a four stroke engine has to breath, You cannot connect a VP30 directly to the crankcase vent. You have to put a tee in the vent line with a restrictor in it.
Enjoy,
Jim
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Paul McIntyre (I hope I remember his last name correctly!), from rec.models.rc.air, posted his experiences with using a Perry/Varsane/Conley VP-30 with an OS FS-91 Surpass. He puts a tee in the crankcase vent tubing and powers the VP-30 from that. He used his while racing in California and says that the impulse from the Tee is more than adequate to drive the VP-30. He also says that the oil drains out of the other side of the tee as is needed for this engine.
Paul is a very experienced modeler and has even designed glow engines with his own brand name on them - Raptor.
Yes, I know it seems unlikely to work, but work it does, according to Paul. I believe Paul without reservation on this matter. Anyone wanting more information on this topic should look up Paul's posts on rec.models.rc.air. They are a couple of years old by now, but are still very relevant.