Engine and Transmission Tech

I’ve got an extreme exhaust backfire issue with my 302 that I’m going to try to resolve and would like any advice anyone has on what might be causing this and how to proceed in diagnosing the problem.
Here’s the problem. When I accelerate hard anywhere only from around 1,000 to 2,500 RPM I get a constant bang bang bang so loud and intense that at first I thought I had something in the drive train hitting, but the banging frequency is related to engine RPM not drive train RPM. When these backfires occur under hard acceleration, if I ease off the throttle yet continue to accelerate but not as hard, the backfires stop. If I accelerate hard anywhere from around 2,500 RPM on up it doesn’t backfire. It idles and cruises down the road fine. No backfires under deceleration at all. It doesn’t backfire when I free rev it. I drove the engine donor car for a while before I transplanted the engine into the MG and it ran strong and great with no backfires at all.
The engine is out of an 88 Thunderbird that I purchased from a Ford dealership mechanic. As purchased it had an MSD 6AL ignition box, MSD distributor, MSD Blaster coil, E303 cam, 1.7 to 1 Crane roller rockers, Edelbrock Performer RPM manifold, Eldelbrock 600 carb, and BBK equal length headers.
I have never moved the distributor – timing hasn’t changed.

Here are the changes I made related to the engine;
Installed a Jegs red fuel pump (same as Holley red) and canister fuel filter.
I installed new spark plug boots and in the process shortened the wires.
I installed a new coil to distributor wire coil boot at the coil and shortened the coil wire.
Installed new spark plugs.
Cleaned and rebuilt the carb – the only adjustment was the float level which I set to spec. The previous owner had the float about as rich as it could go.
Removed a one inch carb spacer and replaced with a quarter inch of aluminum heat spacers.
Replaced the timing gears and chain.
Replaced the the BBK headers with through the fender headers

Here’s my thoughts on what might be wrong;
I have the distributor to ignition box wires in the same convoluted wire bundle with the oil pressure wire, the radiator fan thermal switch wires, and the alternator wires. The MSD literature states the distributor to ignition box wires should NOT be run with other wires.
I might have carbon tracks or something amiss in the distributor cap.
I might have a faulty plug or plug wire.
I might have cross firing between two plug wires.
I might have a carburetor issue - perhaps the float level needs to be set to the same richness as the previous owner had it.
I might have unknowingly changed the cam timing when I changed the timing gears and chain.

My plan is to first check the firing order, check to see if a couple of plug wires are routed such that they could cross fire, visually check the distributor cap. Then determine if it’s isolated to one cylinder by pulling and inspecting the plugs. If the plugs don’t show anything I’ll warm up the engine, set it to run at around 2,000 RPM and check each exhaust tube with and infrared heat thermometer. Next I’ll check the resistance of the coil and plug wires. Next I’ll isolate the distributor to ignition box wires.
Finally if all that fails to fix the problem I'll clean and rebuilt the carb again.

On a side note I only have 118 miles on the odometer since I got the conversion on the road and I'm amazed at the attention and comments I get nearly ever time I take it out. I had a group of eight people looking it over yesterday evening at the local gas station/tavern. Yes, the shop bays at the Marathon station were converted to a tavern a few years ago.

Steve

My WAG would be a failing coil or too large a plug gap.
The MSD fires a multi strike up to around 2500 on a v8. That taxes the coils resources heavily. So a weakened coil has a tough time keeping up. Same thing with a large plug gap. The ignition just isn't up to the task.
I would reduce the plug gap to something like 0.025" and see how it runs. If it's ok then take them out to 0.035". If it's still ok then you found your problem. If it's not good then change out your coil.

Live like you mean it.
Fred

Thanks Fred, I'll give that a shot first.

Steve

Could be a wiped out cam lobe. I've had that happen.

Jim

My Camaro had a bad exhaust lobe. It caused it to backfire thru the carb.