Hi
I recently received a used Max .40 fp. It has a strange addition to its backplate. Someone put a vent in the backplate.
I tried it as is and it runs good at high rpm but if you put down to idle it dies or cycles high and low before it dies.
is the backplate vent a viable addition? Is there a way to tune with this set up?
Thanks in advance for your help.

Harry

If someone put a vent in the backplate, it was to run a pump or pressurize the fuel tank. It will not contribute to the engine's running if it's left open to the atmosphere. You'll wind up with a lower power level of the engine because its pumping action will be reduced, and a tendency for the engine to run lean because it will be sucking additional air into the crankcase. If it was running a pump or pressurizing a fuel tank, the lower-end of the engine is essentially sealed by the pump or tank....that works, but venting to the atmosphere doesn't.

Thanks for the reply Bill.

I had the vent attached to the tank to pressurize it.

today I plugged the crank vent and put in a muffler vent and the engine runs like a dream.

why the cycling when the crank vent is used to pressurize the tank?

When you try to throttle-down with a pressurized fuel tank, the pressure in the tank takes a while to come down, so the engine gets too much fuel. This can cause erratic running, or even quitting, due to too much fuel. A pressurized fuel tank is good only for full-throttle operation unless you have a regulator in the fuel line going to the carburetor fuel inlet.