A Sprint I worked on once we had a problem, It would idle fine. you'd get two or three laps under hard throttle and it would stumble and stall.

We tried everything. Someone told us this tip, Replace the fuel line.
What was happening is the line was collapsing and pinching off the fuel flow.

Don't know if this is their problem. But we thought it was the fuel and several other things, Ours was the line itself.