Holley 750 backfire.

Ajp440 said

It doesn't seem to matter what speed I'm going if I'm easy on the throttle it will go threw the gears and drive like normal. Wether at 5mph or 20 or 30 if I lay into it it spits and pops through the carb. But when I'm sitting in my garage I can snap the throttle and it's clean and crisp.

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Not trying to Hi-Jack the thread but I have the exact same issue right now. It started slowly and is getting worse. Part throttle and it rev's with no misses. As soon as I lean on it harder it gets very rough (missing), almost sounds like it's hitting a rev limiter until I back out of the throttle and it's fine again.
In park it rev's perfectly ! Did it with my 750 Holley (Vac Sec) so I changed to a Holley 650 DP I have and it was a bit better but sure not fixed.
Timing is about 18 at idle, Vac. is disconnected. Electronic Ign with MSD coil.
I'm starting to think it could be a grounding issue somewhere. The hunt begins.
Good luck with yours, I'll let you know if I find anything.