Ok well that’s the problem but it only happens in 3rd and 4th gear (M20) when I do anything more than ease into the throttle. Its more noticeable in 4th gear though and it doesn't do it at all in first and second. I’ve tried it as low as 2500RPM in both gears and each time it will backfire and stumble until it gets to 3000RPM and then it will clear up and run fine.

The engine is a new 406 making around 450 HP.
Distributor is an HEI with aftermarket cap, coil, rotor, and module. It has a stock 25* vac can that I put a stop behind (similar to cranes setup) to limit it to about 12*. I've tried initial/mech. from 12*/33* to 14*/35* and it didn’t make a noticeable difference.

The carb is a 750cfm 3310. It has the orange pump cam and the lightest secondary spring in it.(I forgot the color) I'm not really a carb expert but other than that I pretty sure everything is stock. I did check/set the fuel level and that didn’t help either.

One other thing, the backfiring is worse when I use manifold vacuum than ported. With manifold it makes a slight occasional pop at cruise RPMs(2800). I’m using ported now but if I can get this figured out I want to see how the car runs on manifold vac. after reading some discussions here and at team chevelle.

I’m going to check the plugs this weekend to see if they are fouled up and clean them off. Other than that and what I’ve already checked I’m running out of ideas. Does anyone have any ideas what else to check? To rich? To lean? Any other suggestions? (please no 4 letter words like flat, wipe, or lobe :( )

I'd go back to the plain colored spring to start, could be going lean during the transition.

If that doesn't help it might be a good idea to pull the valve covers and watch to make sure all the rocker arms move up and down the same amount when the engine is cranked (and that none are loose). I had a cam with a **** exhaust **** that caused popping in the exhaust. It also made the exhaust louder and the engine did not clear up at higher RPM.

Eric, thanks for the idea and also for not using any of those bad words. :D

I've been searching over here and team chevelle and it seems cams going flat is a common problem when the engine starts backfiring. I hope since I have a roller that will work in my favor a little but I am definitely going to check the rocker arms for movement now. Its weird how it does it only in those 2 gears and how manifold vac. is worse than ported.

Assuming that the cam is fine does this sound most likely it is a carb problem rather than ignition? Could jet changes make a difference or converting to a secondary metering block so I could change the jets there also?
Any other suggestions are welcome too

I racked my brain for several days having this same problem. Thought it was my msd ignition, carb, timing. A fouled spark plug was my cause.