backfire when load is applied
 

Ok so my problem is that I am having a problem with a backfire through the exhaust while driving and it gets worse with the more throttle I give it. While in park or neutral I can give it as much throttle as my heart desires and have no backfire. I have been driving the car without this issue since may and just a couple days after a test and tune run, boom there it is. I have a 350 with aluminum 2.02 heads, 512 lift cam, 750 hot rod quick fuel carb, air gap intake, msd distributor, msd 6A box. I tried changing the squirter size (discharge nozzle) from 31 to 28, and I dropped my power valve size to a 5 I believe. It has had no effect on the back fire. I don't believe it is an ignition problem because its only with a load applied, but I can't be sure. Any help is much appreciated.
Thanks

Re: backfire when load is applied
 

Backfire in exhaust = unburned fuel and air passing through the engine

Pretty simple really; that's the ONLY POSSIBLE cause.

And in turn, there's only one possible cause that leads to it: an ignition problem.

What do the plugs look like?

When you start the engine up cold and drip water on the headers after it's been running for about 10 seconds, which cylinder's tube is cold? That's the defective one.

Re: backfire when load is applied
 

I pulled only 4 plugs all they were light brown which is good. I'm going to pull the rest. I'll try that drip method but I have used a temp gun to check the headers while running and they seem fine.

Re: backfire when load is applied
 

I had the same problem and it turned out to be my MSD cap wasn't aligned properly and my Ignition control module was bad.

some of the MSD caps have a spring loaded hold down screw to connect the cap to the distributor.. on my distributor, those screws didn't allow the cap to line up properly to the distributor so I removed them and added normal screws.

Also my ICM was not functioning properly so the timing didn't advance correctly causing a backfire..

Re: backfire when load is applied
 

Don't check them "while running"; check them about 10 seconds after a cold startup. One (at least) will be stone cold while the others will have reached a few hundred °.

Cap not aligned will cause all manner of oddball malfunctions; could easily be a culprit here. That ends up sending spark to the wrong cyl ... sometimes. Maybe sometimes not. Maybe 2 cyls at a time. Wouldn't hurt to check that too.