Ok well thatís 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. Iíve 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 didnít 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 didnít 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). Iím 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.
Iím going to check the plugs this weekend to see if they are fouled up and clean them off. Other than that and what Iíve already checked Iím 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