OS 120 Surpass : to pump or not to pump
The VP-30 has a regulator built in. You don't need a Cline regulator at the carburator. I figure the VP-30 would work on the OS 4-stroke, because the OS pump is nothing more than a flexible membrane diaphram that move back and forth as the pressure inside crankcase housing changes. When the piston moves up, it sucks the diaphram forward, when the piston moves down, it blows the diaphram backward. The other side of the diaphram would has a pumping effect that draws the fuel to the regulator. The regulator has a one-way check valve to prevent the pump from drawing the fuel back when the diaphram changes direction.
KrisG, one way to test the pump unit on your OS is to, first, remove it from the engine. Put a small clean fuel line to the inlet nipple, block off the outlet nipple with your finger, and use your mouth to blow into the inlet via the fuel line. If air comes out of the engine crankcase nipple, you'll know that the diaphram is broken.
If there is someone else who has different knowledge on the design of the OS pump, please jump in.