I've seen this happen from restrictive petcocks, lack of gravity head between the tank and carb (a problem on tall singles) and a chunk of rubber from a decayed fuel line lodged out of sight above the float valve.
A friend seized his CB125-engined racer twice before we found out that when Honda designed the petcock on the TL125 tank he was using they apparently decided to make it flow less than usual, because after all how much fuel does a trials bike need?
Larger flow float valves or a mild pressure feed on the fuel line with a pressure-limiting return line to the tank were some of the solutions.
cheers,
Michael
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