One would assume that you you should A) shut off the fuel before shutting off the switch or B) install a new carb and clean out the rest of the fuel system.

tomplumb,

Thanks for your reply. I should have included in my original post that I have already installed a OEM new carburetor, cleaned the fuel lines and tank, and only use "Filtered" premium non-ethanol gas. My engine does not have a manual fuel shutoff valve, but instead has a fuel pump that I believe is activated by crank case pressure. Because I can hear air pressure hissing past the main float valve I know that the system is being over pressurized. I am suspecting that this little external fuel pump is the culprit, but I can't find out how it functions.

If anyone knows how this little pump unit works that would help in determining if it might be the problem. I have included a picture of the pump for reference.

Yes, they are driven w/ a pulse from the crankcase. Inside there is a diaphragm and little check valves The little gold part is a vent. I've not run into one doing that. Seems plausible. Does your engine vent the tank via the cap or does it have a vent hose?

Tomplum,

it does have a vented cap. I pulled the rubber flap out of the cap and made sure the vent groves were not plugged with bugs, etc.. I have even run it with the cap removed and it makes no difference.

i am wondering if the fuel pump runs off crank case pressure if it possible there is a pressure check valve in it that keeps it from over pressurizing the fuel bowl. If so, that could be the problem. Do you know if the fuel pump has such a check valve.??

it only a $20 part, but I have never subscribed to part replacement mechanics as a method of diagnostics.

thanks for any further clarification.