What we have are similar symptoms with the original carb/intake and new holley carb/offy intake. Backfiring (that's getting worse over time) and awful performance, although Holley was better than stock. I verified every vacuum line was good, vacuum readings at idle were decent and pretty smooth. I replaced the intake manifold gasket and used RTV to ensure there wasn't a leak there.
I continue to suspect multiple problems. But this week I was looking closely at my carb setup that has a 1" spacer to prevent obstructions. There's a danged logo/numbering on the spacer which I'd put facing up towards the carb. The carb has some passages between barrels that give more opportunity for vacuum leaks.
On one of the last runs before I tore the engine out, I took carb cleaner to the car. I sprayed all over the place, couldn't find anything. But there was an area when I really doused the primary side of the carb would cause the RPMs to drop. Hmmmm, but I couldn't even see where it was. I thought since the cleaner was dripping on everything maybe some made its way in through the throttle shaft.
Looking at the pictures, it looks like you and those at Camaros.net called it. A vacuum leak.
Below is the carb and the spacer, the spacer is flipped up. You can see on the spacer how the seal isn't made where the passages are discoloration. Look closely at how little metal is there to make the seal with the gasket.
Now look at the primary side of the gasket (rotated clockwise). A bunch of photoshop to try to better highlight the dark streak that blasts right through the "CAT NO. 6006" and on out. Next to it is some weird texture, like it soaked up something (maybe my carb cleaner?). It looks much too much like coincidence.
I'm changing up the carb when the engine comes back together, will either flip the spacer or hopefully chunk it.
Thanks!
Mark
