[tomthecomic]

I have a holley 750 vac secondary. I have had a backfire sometimes in the 26-3000 rpm range. I got an air fuel monitor and if I give mild acceleration it goes to very lean. If I stomp on it it is ok. In partial acceleration do the secondaries open? I thought of changing the spring do you think that is the problem?

[ericnova72]

Check the fuel level in the front bowl. If this is OK, then check how much vacuum you have at Idle. Sounds to me like you need a higher rated power valve(one that will open at a higher vacuum level).
Do you know what is in it now?
There are a few more things beyond these, but you have to start here first.

Secondary spring is not the problem.

[tomthecomic]

the level looks good. I have 13 out of gear and 9.5 in.

[ericnova72]

Do you know what power valve is in it now? Will need to be in the 6.5-7.5 range. What size jets are in the front?
Try turning mixture screws out 1/2 turn each and see if it helps.
Is this a new carb?, old carb? Do you know it's history? Just wondering about mis-matched parts or gaskets, or a rebuild by a previous Holley "hacker".
The stumble and backfire at the rpm you gave is a lean hole in the fuel curve, so I'm trying to systematically find the cause by process of elimination as different circuits in the carb overlap each other much like the shifts in an auto tranny.