I instruct all of my customers to ALWAYS run their petcocks in the reserve position. Petcocks obtain their fuel from the lowest level in most fuel tanks when positioned in the reserve position. Petcocks obtain their fuel from a stand pipe that may be 2 or more inches from the bottom of the fuel tank when placed in the on position. High performance two strokes that are highly tuned, can burn a piston in less than a couple of seconds or so when the air fuel mixture becomes too lean.
I have seen a large number of pistons torched on engines that had the petcock positioned in the on position. The fuel level in the float bowl does not stay consistent when the fuel tank level becomes low enough to not keep the stand pipe on the petcock completely submerged 100% of the time.
As the fuel level in the fuel tank becomes lower and lower and the fuel is sloshing around, the stand pipe is not providing fuel to the float valve 100% of the time. The inconsistent fuel supply to the float valve causes the fuel level in the float bowl to stay lower for longer periods of time until the lean condition causes a piston failure.
A two stroke will need a fuel flow rate through the carburetor of at least 6 gallons per hour when developing around 70 hp. You will never use 6 gallons per hour if your engine develops 70 hp but the fuel supply past the float valve must be at least 6 gallons per hour any time the engine is developing 70 hp to keep it from going lean for the period of time it is at that power level.
A flow test with a full tank will provides a lot more fuel pressure and flow, so always perform this flow test when the fuel level in the tank is 1 to 2 inches from the bottom of the tank.
