I have an O.S. MAX FS .61 engine set up in a test bench. The first time I tried to start it turned over on the second slap of the prop. Ran great! I shut it off. Made some carb adjustments and tried to start. Nothing! Notice that when I opened the card just a little bit that the gas would drain back down the tube into the tank. So, I close the carb, pressurize the line and quickly opened the carb and started the engine. Is this normal? You can see the bubbles and the gas drain back each time I try to start it. Thanks for any help.