I've got a 63 Panhead that acts like it's running out of gas. Fresh engine rebuild, went through the Linkert M74B, new isolation block, new rubber ducky float and new float valve and seat. Float is set as recommended. Steel line running from gas tank through original fuel screen. Two priming kicks, key on, and it starts every time. Engine sounds good. When we get down the road about 8 miles it starts missing , some backfiring through the carb, and just generally acts like it's running out of gas. If we pull over and give it a minute, it takes off and runs OK for a few miles and does it again. When we first fired it up and ran it the first 75 miles it didn't do it. I cleaned the fuel screen, checked for gas flow, took the carb off and looked for obstructions and re-checked float level. checked for intake leaks. It only does it when the engine is warmed up. Any ideas? Thanks!