Malcolm,
In your original description of the problem, you stated that:
"The fuel system is not stock. At some time in the past, it was converted to the two tanks on each side supplying only the engine on their respective side. The supply tubing goes from the bottom of the lower tank to the blower-driven fuel pump (vane type) to a single fuel filter to the injection gallery on the cylinder head. Return flow is from the return gallery on the cylinder head to high on the upper tank." If this description is correct, it would appear that no connection exists between the return lines of the two engines. If true, then your issue cannot relate to a fuel return valve problem.
You also stated: "there is an interconnecting balance tube running from the bottom of one lower tank to the other to keep the tanks at the same level." Where is this connected? Is it into the rear ports on the in-tank filter housing? If so, potentially the left in-tank filter is clogged and the left engine was partially or completely running on fuel cross-feeding from the right tanks.
If your original description of the modified system is correct, it sounds like your issue is that little or no fuel was being drawn from the left tanks, but excess fuel from the left engine was being spilled into them.
Colin
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