Thanks
The return line is a very small line ,way smaller then the fuel line so I think that part is OK but there must be NO check valve in it or it couldn't be returning fuel that lets the engine run when main fuel shut OFF.
On the rest of your observations those are all good points .I shut the electric fuel pump off so it is only getting fuel from the mechanical pump and it still forces fuel out the overflow. So that eliminates the to high pressure electric fuel pump.On the float/needle the fact that is runs perfectly when getting its fuel from the overflow would seem to show that those are not the problem.Also the carbs have cable supports on them to help with vibrations.
I will locate a check valve to install.
THANKS AGAIN