AD 10 Sneeze

[yooperrigged]

Hello. After a few minutes of idling my AD 10 will sneeze and lose pressure from the tank. I have switched tanks and the same problem occurs. If I pump the tank it will run fine again. I cannot see any fuel leaking anywhere. The gas lines are original so I mmay be overlooking the obvious. Any suggestions? Thank you in advance. Dan

[Randy in Tampa]

Definitely sounds like a pressure leak to me, I would separate the tank and pump it up see if it holds pressure, if it does it’s on the motor side new fuel lines would not be a bad idea ..🤔

Rotary valve Johnson’s Rule!

[frankr]

After the motor runs a couple minutes there should be pressure in the tank, which will give a satisfying hiss if you loosen the cap. If that isn’t happening, there is an air leak somewhere. The old soapy water trick should find it. OR- or it is possible that the motor isn’t pumping air into the tank. Special rubber valves on the reed plate ahead of the carburetor supply that pressure.

[yooperrigged]

Ok so with the tank disconnected from the motor if I pump to pressurize, should fuel come out?

[frankr]

Pumping the button does NOT pressurize the tank!! It only operates a small fuel pump within the tank, which pumps fuel to the motor to fill the carburetor so the motor can be started. Once it is running, the motor pushes air into the tank which pressurizes it. Then the pressure in the tank forces more fuel to the motor so it can continue to run.

[fleetwin]

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”.

[yooperrigged]

Whew! New O rings on the connector and the problem is solved. I guess it made sense to start with the simple stuff first. A sincere thank you to all for your help.
Dan

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