Fuel Fluke

My Suggestions Would Be As Follows:

1st remove he fuel lines to buy-pass the mechanical pump and try running the engine with only the electric pump. If this takes care of the problem then the mechanical pump is bad and may even have a leaking diaphragm. In this event you should also make sure it did not leap gas into the crank-case. Check you oil level and smell the oil to make sure there is not gas in it. If there is drain the oil, buy a cheap oil filter and low cost oil, do the oil change and run the engine for about ten minutes and do another oil change with a higher quality filter and the oil you would normally use and you should be ok.

2nd if the mechanical pump is not leaking into the crank-case and electric pump along did not shove he problem then switch the process and remove electric pump any run it on only the mechanical pump.

3rd if neither of these fix your problem then you should check you needle seats and float levers.

Good Luck!