Help! I have just installed a new 355 Crate motor, RPM Heads, Holley Avenger 670 carb, MSD Billet Distributor. This is in a 1972 Chevelle. Problem: Carb was jetted for old engine, had 70 and 73 jets, put original 65 and 68s back in. Car ran with 70's ok, not great, but it smelled like fuel and would load up and backfire. So first time I started with original jets, I am getting popping noises out of the carb. It doesn't sound like backfiring, more like repetative popping. Vac gauge shows 13, fluctuates to 11, then back to 13 for a little. Do you think it is a vac leak causeing this? Suggestions for best carb gaskets and torque for carb nuts. I have a 1" spacer and the Fuel regulator adapter plate inbetween manifold and Carb.

The popping in your situation is caused by a too lean mixture. Not real good at Holley carbs, but adjust the accelerator pump, and jet up.

Thanks for the info! I had called Holley and with the specs I gave them, they felt the power valve needed to be changed to 4.5 and jet up the carb as you stated. After performing the above, I took out the plugs and inspected and put in new ones. Two of the plugs were dark black, the others were light brown. After all was in and firing order rechecked, I tried to start the car. AGGGHHHHHHHH! graemlins/angry.gif The starter was bad. It just spun, no engagement. Time for graemlins/beers.gif

Problem Corrected! graemlins/hurray.gif
After shimming the starter, I marked the Harmonic Balancer and went through the valve adjustment procedure outlined in tech tips. Great Stuff
I then reset my timing, adjusted Fuel/Air again, idle, and all is great. Damm, this thing has a little umppph. Now I have to be patient and wait for a few thousand miles before Dyno and rompin. Can't wait. Thank you to all who responded. This site is invaluable, a lot of great advise and tech stuff. Cheers to All graemlins/beers.gif