What is your fuel pressure? Most likely scenario is that you need to replace the fuel pump. I run mine from full to less than a gallon remaining regularly. No performance changes at all should take place after a quarter tank.
They do, it is very common to have fuel starvation at fuel throttle below a quarter of a tank. If he is bogging down at 50% or 100% then I would say fuel pump. But pressure is going to change below a quarter of a tank.
Plus below a quarter of a tank you are burning your pump up and causing it to wear out prematurely. Normally it is able to be cooled off while it is submerged in the gas but without that it is overheating.
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