If you're having trouble from hitting the gas when the truck is loaded it says one thing for certain...by doing that you're driving the manifold vacuum down low. Probably into single digits. That will naturally drop out the timing advance provided by the vacuum biscuit but there's something else at play. I think timing is a factor but maybe not the only one. Let's consider if you had a Rochester with a working power circuit (rare, sadly) or Carter YF with a working power circuit (less rare) dropping that vacuum suddenly would cause your air/fuel mixture to go almost instantly rich. Very rich. In those two carburetors it happens this way: there's a spring that constantly is in play on the power piston in the Rochester and on the diaphragm (and coincidentally the metering rod) in the Carter. Vacuum pulls against the power piston or the diaphragm and that keeps the power piston from opening the power valve in the Rochester and it pulls on the bottom side of the diaphragm in the Carter which keeps the metering rod in its lower range (the leanest range) in the Carter. Deny either of those carburetors vacuum and you get a much richer a/f mix instantly. Why? That's how they're designed. That is so your engine gets the fuel it now thinks it needs to pass another vehicle, climb a hill or move a heavy load. Now in the Webers you've bought I've never studied their power scheme, however as I recall it works with a diaphragm (vacuum controlled) and a regulating spring. My guess is your first problem is running the engine a bit too far advanced. Since I don't know for sure I'm going to assume your vacuum advance goes to ported vacuum and not straight to manifold vacuum. If I'm right at idle if you disconnect the hose going to your vacuum advance biscuit there will be no change in timing...ported vacuum won't provide (or should not provide) any vacuum to that biscuit. And here I'll add it is important to set your timing at idle using the factory specified idle speed...because remember speed and rotation (centrifugal force) are what make your mechanical advance operate. Run your idle up too high or down too low and your timing will not be what the people who designed your engine wanted. Even 100 rpm one way or the other will cause a difference. Anyhow back to your carburetors, I would start studying them to see what makes their power circuit tick. If you're getting backfiring only out of one, then there is a reason. Something is causing the front carburetor to work differently than the rear one (obviously). Please remember too lean of a mixture and too rich of a mixture can cause backfiring. Too lean? It will burn more slowly...possibly too slowly. Too rich? All of it may not burn before the exhaust valve opens. Either way unburned gas goes where you don't want it and can ignite. In my experience backfiring through the carburetor is nearly always caused by an overly lean condition. I've even seen this occur as the result of a bad accelerator pump and since you have 2 carburetors this might be your answer...one pump is okay and one is not working. You hit the gas and one of the accelerator pumps doesn't shove any gas into the manifold...just a bunch of air and what gas normally follows through the venturi and then "bang" goes the backfire because the mixture is wrong. Good luck. Hope some of this helps.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end