Chrysler LM318 gas engine problems

[duane]

I have a single screw 26 ft trojan with a chrysler LM318. At fast rpm (2800rpm or faster) once in a while it backfires through the carb. This is a terrible loud noise that you dont want to hear. Below that rpm, it does not seem to do it. Also, only cylinders 1 and 2 (front left 2 cylinders) are the only ones that are buring RICH. I replaced plugs and after only 3 hours these two plugs were BLACK. All the rest of the plugs look good. I had both heads rebuilt last year. Yesterday, I replaced the Distributor cap, rotor and the two spark plug leads for 1 and 2. I have not dared to rev up the engine again after this noise. It has happened before also. Does anyone have any idea as to where to look. My next check is going to be to time the engine and also relook at plugs 1 and 2. Since it only effects no's 1 and 2 it seems to me to be associated with the head gasket, valves, or spark plug leads. Any suggestions?

Thank you

Duane

[jehardiman]

It is cylinders 1 & 3 btw...

Anyway, Since it is only those two cylinders and you replaced the plug wire4s and cap, I would next check the carb floats, metering rods, and needle screws. Bad fuel or low vacuum can cause sticking which is allowing raw fuel to pool in the manifold and enter that runner. Does the carb need/have the proper carb wedge?