by Had Robinson
updated December 18, 2021
There are two situations when stall occurs. The pilot bends over at launch and the engine stalls. It is caused by the puddling of fuel and oil in the crankcase. Bending over will allow the flywheel to slop the puddle (usually with a higher concentration of oil) into the combustion chamber. The engine may either smoke a lot for a moment and/or stall. This is not a repairable problem. It can be mitigated by not idling the engine excessively.
In the other situation the pilot is at launch and ready, glider overhead, moving in the right direction, and goes to wide open throttle. Rather than surge ahead with power and away from the ground, the engine stops or fades, like happened to this pilot. The glider can wind up on top of the pilot and the motor. It is probably one of the most annoying and embarrassing events we motorheads experience. What happened?
It is called vapor lock and is caused by a negative pressure on the gasoline in the fuel system. A practical fix for this problem is to apply the throttle slowly and deliberately when launching. This will keep the pressure in the carburetor from dropping too fast which caused the liquid fuel to become gaseous. For more information on vapor lock and how to prevent it, go to this page.
It is also a good idea to rule out problems in the fuel system which contribute to stall. Do this quick and easy test to rule out acute fuel starvation due to defects or problems in the carburetor and other parts of the fuel system.