Reply To: AD 10 Sneeze

[Don]

You say that the same problem occurs when you switch tanks, I am assuming each tank has its own fuel/air lines and connector….In other words, you are not using the same fuel/air lines and connector on both tanks….
Usually, you would see/hear an air leak in the small hose that runs from the intake manifold to the engine end of the fuel line connector. Does it look like someone has had the intake manifold off, do the gaskets look like they have been changed? Is this engine new to you? If not, when was the last time it ran properly? I suppose it is possible that both tanks/lines are messed up, but that doesn’t seem likely.
Once the engine is running properly, pump the fuel primer button up, then remove the fuel line connector. Fuel/air spray should be coming out of the engine connector on the air side. If not, perhaps, the return line or connector is plugged up, not likely but possible. Otherwise, there is a problem in the intake manifold (broken rubber check valves like Frank says), or improper/messed up gaskets. The two intake manifold gaskets are different, so swapping them will cause a problem, or perhaps using two of the same gaskets. If all this is OK, then the engine might have an internal problem that is preventing it from developing the crankcase pressures needed to operate the tank properly. But, in this case, the engine probably would not be running correctly.
Look for the simple things first, before “getting in too deep”.