[Jerry Clayton]

Couple of things you need to clarify----top of 3rd or 4th---which????same gear all the time, most of the time or completely random???? Each and every time on the run up thru the range?????
Pedal on the floor----do you mean that you don't lift to shift????
Is it a vacume seconary carb????

Do you have a rev limiter in your ignition and at what rpm???

Now---other than those items---have you tried this carb with center pivot float bowls???


IFIFIF ( if )

You have a vacume sec carb, rev limiter, don't lift to shift----you will occasionally get a backfire from any or all---over rev will cause the seconaries to close,changing mixture, and interupting the ignition randomly( so this could be random misfiring cylinder lighting off unburned fuel )

With the rev limiter cutting ignition, ubburnt fuel mixture is passed to the exhaust,where the heat ignites it, causing a backfire back thru the intake while valves are on overlap

When an ignition rev limiter drops cylinders, and then refires--it can cause a crossfire thru parallel running wires to an adjacient cylinder---This is common on Fords left side bank--how are your wires run????

Does your carb use seconary metering blocks or metering plates???

Jerry