[Jon Bradshaw]

Malcolm

I have a similar issue when operating my ferret. After sitting for a winter the ferret loses prime in the fuel system so rather than sit and crank the batteries I fitted a small auxiliary tank above the engine bay to get it started. Once it gets going the fuel from the mechanical pump takes over and keeps the ferret running. The fuel tank is above the engine bay hanging on the back of the turret and will quickly overflow with fuel back flowing from the carb under the pressure of the mechanical pump.
My thought is that one of your fuel pumps is too strong and actually pulling from one tank and pushing the back flow into the other rather than through the engine. The cross leveling tube is likely too small to handle the amount of fuel being pushed up.
The trouble shooting tests I would do are-
With the left tank very close to empty, clamp the cross tube off, fill the right tank and start the engines, look into the left fuel tank and see if fuel is bubbling in from the lower (pickup) line.
You could also switch the fuel tank feed lines to the opposite pumps and see if the opposite tank fills up.....

Either way you are driving a working Sherman so good times!

Jon