losing power with century short shaft

[wdf]

okay, this is engine no.1 and the oldest, the 1961 century.

Took it for a spin today. (first run after rebuild) and it runs fine, but after a time at max power, it occasionaly dies down a little. after reducing the revs, and then opening the throttle, it is back to max power, but then dies down again to maybe 2/3 to 3/4

it appears this is some sort of fuel problem, but everything is clean, the in tank filter is perfectly clean, and the carb is clean.

any thoughts??