A lot can depend upon how the carburetor was rebuilt. The only proper way is by complete diassembly and soaking all of the components in a bucket of carburetor solvent for a few hours.
After soaking, the components are rinsed out and blown out thoroughly with compressed air.
If the accelerator pump discharge nozzle is Y-shaped and fuel is not being discharged on the passenger side when the accelerator is operated then some debris has likely plugged the nozzle.
At idle, looking down the 2 throats with a bright light, you should see no fuel spray or dripping at all… When you quickly snap the throttle momentarily open, you should see an equal spray of atomized gasoline coming out of the center of the booster venturies along with a squirt from the accelerator pump…If only one throat is drawing fuel, then the other side is plugged up and starving 4 cylinders for fuel, causing the backfire…
With the mixture screws 1 1/2 turns out, if the engine idles acceptably well the float is likely OK.
The heat rise valve may be stuck in the closed position and causing problems. Also, the EGR valve and intake gaskets may be a problem. I recall a thread here regarding an RV with such problems.
When the carb was rebuilt was the float replaced? The old brass floats were good forever if they didn;t spring a leak but the neoprene ones eventualy absorb gas and get heavy and don’t shut the needle valve off.
On my Ford truck w/a Motocraft 2150 carb, if I’m suspicious about the mixture and there isn’t anything else obviously wrong, the first thing I do is measure the fuel level. That’s fairly easy to do. Sometimes that gets out of adjustment, and the mixture is sensitive to fuel level. I have this gadget that you place in a certain spot across the carb, then you slide another thing up and down 'til it hits the very top of the fuel. I think it came in the package with the carb.
When the fuel level is too high, it has either been that the float mechanism is out of whack, or that the inlet valve where the gas comes into the carb depending on the float level is leaking.
well lady, you seemed like you know your way around your truck, have you figured it out yet?