I’ve had this problem for years with all of the ’28 Quads I’ve ever owned. The top carb runs out of fuel at anywhere of 1/4 tank and up to 1/2 tank of fuel left. This has happened on 3 different ’28 quads I’ve run. As I run them I have to keep an eye on the top carb float needle to see when it starts to not keep carb full of gas. Motor will still run on all four cylinders until float needle is at bottom of it’s travel. Maybe Gary knows a cure?? It must have been a problem back when they were new and I’m wondering if Elto ever came up with a fix for it. You can’t raise the tank because of it’s design and I wouldn’t want to anyway.