What size I.D. is your current fuel line and gas tank pickup ?
Fuel gremlins come in many forms
The smallest hole in the system is the biggest restrictor to fuel flow
You may want to test that system another way at the track, jet the carb up one size at a time to see if you have enough fuel volume to that carb to slow the MPH down If you can slow it down by jetting it richer you have enough fuel volume delivery for your current power
If not fix it