Hey delta,Id be real suspicious of the fuel lines. with your problem isolated to the rear tank setting I would replace the fuel lines to the rears at least. Then cut them open to see if one or both had delaminated from the ethanol/fuel issue. The delamination will eventually collapse pinching off the fuel flow. (not visible from outside) also as mentioned the primer bulb can suffer the same fate but since it works on main tank setting, I would guess either the valve is clogged or the fuel lines are delaminating. Bob