anyone know what might be causing it? I'm thinking it's my timing but not sure because the guy who timed it says he has it at the 6 degrees that it's supposed to be set at, atleast that's what it says on the hood. it's a 305 TPI... plmk what i can do to fix it

 

More car history and:
Other symptom besides backfire.
When does it do it i.e. freerev, coasting, part throttle...etc., is it consistent?
Has it overheated at anytime.
What did you before it starting doing it, and what besides timing have you done.
Was the proper procedure following for setting the timing.
Have you checked for vacuum leaks?
Anything in the vacuum system like old hoses, diaphrams, or small valves, can be damaged by repeated heavy backfires, and adding to the leak.

 

it backfires while driving... not idling, it has overheated in the past but only like twice... before it started happening my MAF died... after replacing it i had a boat load of problems... so i gave it a tune-up, i put it on a smoke machine to test for leaks and it had none, replaced the distributor cap and rotor, not sure about the timing as i've never done it and my mechanic did it for me

 

One quick check, pull the fuel pressure regulator vacuum line off after a go around the block to check for a fuel leak in the regulator diaphram.

If you're entering an incline ramp to a freeway is the backfiring worse?
If so connect a fuel pressure gauge to the fuel rail and tape it to the windshield. At idle it can be between 30-40psi, as you accelerate vacuum decreases causing the fuel pressure to increase. WOT should be about 43psi, but you may not to do WOT with it backfiring.

The fuel pump is going bad if the fuel pressure drops at anytime during acceleration, be sure to have someone monitoring the fuel pressure while you drive.

As to the timing; I'm sure your mechanic did it right, but just for the knowledge ask him to show the Setting Connector, there's no excuse for him not to, if he can't, or won't, I'd find another mechanic. I can tell you where it's at, but the point is does he.

 

The two times I experienced backfiring through the carb, I ended up with a couple of wiped intake lobes on the cam shaft. I had two cam shafts that did this and both were replaced by a GM tech at the local Chevy dealership. Bad cam cores is what happened, soft .

 

Same thing happened to me and it was the cam.