Good evening all
I have fought this backfire through carb since I bought this in june { my first corvette } car had not been driven much the three years before I bought it, previous owner was not sure exactly what the motor is, I did talk to the shop that put the aluminum heads on for po, he remembered the car and thought it was a 355. first thing i did was get old gas out and run several tanks of good gas through it. I guess I should tell you everything I know or dont know about car.
355 cid ?
650 Holley, model 4150
aluminum heads and intake, no names or identifiers on either
performance cam I assume due to the way it idle's and i can get only about 6" vacuum
headers
4 spd.
when i got car, plugs and inside of carb were black, rebuilt carb with holley kit, changed power valve from 6.5 to 3.5,, installed new plugs, wires, new distributor { old one was worn out}, adjusted valves per Lar's sheet, set timing to 16 initial and 36 overall and all in by 2800 rpm.
If you are easy on throttle you can get by without the backfire, it is when you start to get a little aggressive with throttle it backfires and seems it does it with less throttle the higher gear your in, it will only do it with load on it.
I have been a mechanic my whole life, worked on everything from lawn mowers to semi trucks, rebuilt engines, carburetors, transmissions and differentials. i have never worked on anything that has been modified, so this has been a little bit of a learning curve in respect to that.
I been reading this forum looking for clues, there is lots of good stuff on here and a lot of knowledge. ANY HELP WOULD BE GREATLY APPRECIATED. Thanks in advance

 

I'll take my first swing at it,,, verify you timing is really where you think it is. Physically check #1 TDC, then verify mark on balancer.
And if you haven't checked for vacuum leaks, I'd go there next.
.
Thats a basic starting point,,,,,,

 

A true backfire means a plug fired while an intake valve was open - 100% timing

 

I had a similar problem once. It was a flat camshaft lobe.

 

Thanks for reply, I did that, compared TDC against timing mark and rotor position, all good. unplugged all vacuum from carb and intake, sprayed starting fluid around carb and intake while running, could not see any difference. The only thing I can think that I haven't checked is fuel pump pressure, I've read that lean condition can cause backfire.

 

Add four jet sizes to the primary.....a 650 will come jetted quite lean for a 350 with heads, intake, cam......and see if this helps or cures it. If it does, add four to the secondary.
Get the model number off of the choke horn and post it.......unless you recorded what jets it had in it when you had it apart.

Jebby

 

Thanks everyone for reply. Jebby I thought I had taken pictures of jet sizes when i had carb apart, but apparently not, but i am sure primary was 68, secondary was maybe 72 or 78 not sure