vun fifty driva.
on the marvel schebler carby there is an accelerator pump.
in the linkage is a little triangular plate with 3 holes in it. the holes have different distances between them.
when you push in the throttle the little pump gives a squirt of fuel up into the manifold. how much of a squirt depends which way around the little intervening triangle is positioned.
if the little triangle is set up wrong you will probably get too much of a squirt.
the other option giving not enough squirt would probably be picked up straight away when the carby was being worked on.
too much of a squirt is subtle though. you really only see the way over rich mixture cut out when you push the throttle in suddenly. push it in gently and the engine may flag a little but will keep on running.
I know about this because I encountered it when I reconditioned my carby.
I eventually got it completely right.
I'd suggest that the carby may not be set up quite right. easy circumvention is to just gently increase the power each time. don't slam it to full throttle.