I have 2 saito 150s . One is in a 1/4 scale Citabria with a slimline pitts. The engine is mounted horizontally(sideways) so I have no issues with the fuel tank. I just checked my needle setting and it's at 4 and almost 1/4 turns open with a Mejlic 18/6 prop. My other 150 is on a Golberg Sukhoi with the fuel tank over the c.g.(about 7 or 8 inches away from the engine) Therefore I wanted/needed a pump (Perry 4-stroke). I have to run the pump WIDE open to get it rich enough. It runs fantastic with the pump but took me awhile to figure out the pump needed to be wide open. In fact I had to modify the pump mount to get it out furthur from the centerline so it could occillate more. Not sure but I think that check valve and tank position may have something to do with your problem.