Try pulling back on the choke lever next time you floor it and see if that does anything to "fix" the problem.
If it does, then you most likely have a bit of grit or something in the main jet and it will have to be cleaned out.
Oddly enough this seems to happen a lot after rebuilding everything.
I'll try the choke, Cleaned the jets, Have new tank with av fuel and no pre filter.
Funny how at full throttle, stumbles, but at 3/4 throttle works fine, with good power
Thanks, working on it toady
Bob Boltz
Also make sure you have a fairly full tank of fuel.
My Isetta would never run as well flat out when it was on Reserve as the gravity-feed system could not keep up with the demand.
And last: do you have any kind of pre-filter installed in the feed line? If so, get rid of it.