IF your running a Holley it sounds like accelerator pump is to small,Holley's and Automatic transmissions will at times Stumble just off idle.most peaple think right away that carb is to big so they try a new one.don't no how hot your engine is but even a mild street motor can handle up tp a 750 with vacume secondary'.when you punch it the car falls flat on it's face "waaahhh",thats called a hole in the fuel curve,tranny's putten major load on engine at(WOT)and the engine is running out of FUEL,to fix this problem if you run a holley, install a Holley(REO-KIT)this is a 50cc accelerator pump kit you can also try larger pump shooters and fine tune pump cams. also check to make sure the factory Bronze Holley Fuel filters arn't plugged this will also cause a fuel shortage.filters are located behind the fuel inlet where it screws in to fuel bowle.I yoused to take mine out and run a in line fuel filter,you will have (2) filters in carb bowles front and back if you run a dubble pumper carb,Also check Float level.the hole Holley kit is like $40.00 the 50cc Diaphramis only$12.00 if you start with just the Diaphram first.A new Holley 750 double coast around $400.00, so any way try that if all is well with the timing and problem still persist I hope you get it squared away.