Car hesitates and backfires when I start moving from idle, sometimes even stalls. Seems that I hear the backfire from the exhaust, when it stalls it definitely backfires through the carb. When I shut the warm engine down after some intensive driving, it backfires once through the exhaust.
It works nicely once I manage to get it moving from start.

It is a 350 with stock bottom end, Trick Flow 30400001 heads, Edelbrock 2102 cam, Weiand 142 blower and Holley 4165 (6210-3) 650cfm double pumper carb. It is a spreadbore carb with mechanical secondaries. I have jetted it up 5 sizes in primaries and 8 sizes in secondaries from where I was before installing a blower, to keep it in safe side.
Carb is not boost referenced. I know that 650cfm is not enough for a blower and I am probably losing some power.
The ignition is set to 6 degrees initial advance and 28 degrees total, the vacuum advance is not currently connected at all.

I have tried changing power valves from 6,5 to 8,5 to 10,5 with no difference. Unfortunately I do not currently have a possibility to check the AF ratio under load. Where should I start from before melting the pistons?

Upgrade 30cc primary side accelerator pump into 50cc? Boost reference the carb? Change jetting? Increase the size of the power valve channel restrictions? Replace the stock fuel pump with an aftermarket one? Check the ignition instead of fuel delivery? Any other ideas?

Thank you!

 

What do your plugs look like. You may have richened up the primaries too much.

 

I'd say you need more initial timing. I would connect the vacuum advance to manifold vacuum and see how it runs.

 

I haven't read all the post or the answers but I set up TWO blower cars this month. In both cases backfire and surging was a problem.
In both cases richening the jets solved the backfire problem and I went up 10 sizes in one demon and 5 sizes on my pair of 850 and all backfiring/hesitation is gone.
Surging?? I forgot about what a timing light said and just started advancing the timing a little at a time. The motor really liked more advance and before I was done I was at 43 degrees and the motor is quit happy, no surging.
Start with richening the mixture 4 or 5 sizes and see what the motor tells you.

 

I dont know exactly what your doing with the carb, but your lead is too low. A couple of ways to clean the engine up is to increase the overdrive on the blower for more air, and also to increase the lead to at least 36 and go from there. It may need more than that. As the lead increases the idle RPM`s should also go up, then just turn the carb idle down. It should be no higher than a normal non asperated performance engine.

 

most of the blown motors i have worked on have 18 degrees initial with 30 to 32 degrees total in by 2800 rpm, i always suggest a blower carb with boost referenced power valves since the supercharger is a air pump, this causes a stock power valve to see the supercharger created vacuum instead of the engines actual boost or vacuum. a boost referenced power valve is connected to a tube below the supercharger so the power valve works properly.most times we also have to increase the carburetors pump squirt on a blown motor.

if you want more to read on tuning a supercharged engine go to www.automotiveu.com