Super A Gives a Loud Bang Through Muffler, Won't Start

Excuse the long post, but its all necessary to explain the problem!

I'm stumped on my 1948 Super A. About a week ago, the Super A had been running for about 20 minutes. I had it pushing in a John Deere M (no comments, now) that I was rebuilding. I stopped to kick the block from under the Deere, while the Super A was running about 3/4 throttle, with a snow blade, so there was load on the Touch Control. I got back on the A, and gave it a little throttle to push the Deere. From the three seconds I had gotten off and back on the Farmall, it suddenly had no power. As in nearly stalling while moving in first gear down a slope. I cut it off and replaced the D21 plugs with fresh. It ran much better, well enough to push in the Deere (which was quite a load) and even scurry down the road in high gear. It was still sputtering slightly, but it had power.

I pulled the Farmall out a day ago to (once again) move a tractor. It started immediately, I am hand cranking, but it had no power, once again. It died while I was trying to ease it into an area where I could work on it. It wouldn't start back up.

I tried to yesterday, cold, and all I got after four hand cranks (choke at 1/2 like always) was one loud bang in the muffler/manifold. Enough to loosen the muffler from the exhaust pipe! No backfire, but just one loud bang. The engine didn't even try to start.

I have checked timing. Everything is in tip top shape: fires #1 right at TDC. It can jump a 1/4 air gap at the plug. All the valves are free and adjusted properly.

The carburetor is spotless: good fuel flow, no junk in the lines, clean jets. Before this happened, the tractor would idle low enough to count the fan blades. It is a Zenith with no high speed adjustments. Compression is good, equal within 5 +/- across all four cylinders.

I also have good clean fuel, I drained the tank and refilled. All I still get is a loud bang.

So the question is, what does the problem seem to be? I have worked on quite a few engines (but not as many as the folks on this list) but I've never heard such a bang without a hot engine and too much choke!

Thanks for your help!
Ben W.

Have you checked the points and coil if the points is arking fire or the coil is breaking down would cause the problem

Yes I did, forgot to mention that I have a loose rebuild H4 magneto that I have for sale that I plugged into the tractor. It gave the same result.

Ben W.

Sounds like it is firing between ex & in rather than on compression with 2 wires crossed.

A bad distributor cap with a carbon track, or a physical crack that re routs the spark is my guess. If you used the same cap and changed mags I stand on that. If you changed caps with the mag and put the wires in correctly It needs more diagnosis. If the timing needed no adjustment to be on TDC, the gears are probably OK as they do not get better. The no power event leads to thinking about missing due to spark issues Jim

check your rotor my cub pulled that once it was all black and hole was almost burned through it

Ben,
Check the wires for firing order, the distributor cap for cracks, moisture, check the rotor, make sure the greese is where it is suppose to be and not on the contacts.
Bob

Unless he has gremlins, nobody changed the plug wires around while the tractor was running, while he was standing there.

Backfiring is a timing issue, and timing issues without deliberate timing changes (you would KNOW that you changed the timing) could mean a major internal problem.

It's not a huge deal to pull the valve cover and verify that you don't have any sticking valves, loose tappets, and that your valves are opening and closing properly.

Long shot here, but check your oil bath air cleaner cup for ice/ water. Could be you aren't getting air.
Our A had ice in the cup one winter when we had lots of snow. Used daily to take hay out to the cows. We figured the snow was getting separated out in the cup, then on a warm day it would melt--then cold night--ice in the cup. Blocked the air flow completely.

All the valves are free and adjusted. But do you have good oil flow to the top end (valves) so they don't stick. When you pull the cover the top end should be wet with oil running down the rockers to the valves.

the role pin in the magneto gear is not broke is it