Farmall A backfire

It has been a long time since I posted and I having a bit of a problem. I have a 1947 Farmall A and it has the wrong magneto on it. It was that way when I bought it. It has the Mag off of a cub on it. I purchased a used magneto that was correct, but after installing it I found it didn"t have much spark (it had weak spark off the coil, but none out of the distributor)so I took it back off and reinstalled my working magneto that is the wrong one. Now I have popping through the intake, it is also making a gurgling sound in the oil bath. What could be going on here? Since this is the wrong magneto my service manual tells me nothing about it. I followed the set up precedures for the correct mag. I also installed a new manifold. The previous manifold had an exhaust leak and their was no vacuum through the intake stack. With the new manifold I have plenty of vacuum though the stack, but will get a puff of "air" out of it between sucks. I don"t think that the new manifold would have any effect on the ignition timing, but I figuered I tell you about it.

The magneto that was on the tractor has the 1 position at 12:00, the correct magneto has it at a differnt spot.

Bryan, are you sure the mag is timed to #1 cyl on the compression stroke ? It sounds like it maybe on the exhaust stroke.

I think so. The manual said to find the DC mark on the flywheel and line that up first. There was no DC mark on my flywheel. However there was a large mark that came a point on the flywheel. It looked painted on. The was a letter C at that location too.

If I didn't spin the crank around twice I could have done that. I didn't turn it twice, I only did it once and thought I was on the compression stroke.

One way to be sure is take out #1 spark plug and turn over the engine, check for compression by putting your thumb over the hole. Once you're on the compression stroke use a slender rod through the spark plug hole to find TDC as you rock the crank back and forth. then mark the crank. Hope this helps.

I took off the valve cover to make sure both valves were closed. I then spotted the D-C/I-4 mark on the flywheel. I realigned it and runs about the same. When it popps through the intake it blows out a bit of oil from the oil bath and then blows out a puff of black smoke from the exhaust. I don't get it. I obviously had it set wrong before because I lined up with another mark on the flywheel. I am amazed it ran at all the first time.

Only one other thing to check that occurrs to me and that is to check the spark plug wires at the Mag to be sure they are in the right firing order.

If it was on the ex stroke , it wouldn't run at all. Check the firing 1342 clockwise looking at the cap. If that is right, put in new plugs.

The mag must click (the impulse coupling) when it passes the timing mark.
Set the engine pointer just before TDC on the F/W 1/2" or so, but moving it in the correct direction only, and not going past.
Loosen the mag in its slots and turn it till it is retarded (CW) as far as it will go. Then set the engine at TDC going forward only. Then turn the mag slowly CCW until it clicks. lock it down. This cocks the mag impulse, then times the release to TDC. When running, it is advanced to the correct position. When starting it must be at Impulse TDC. JimN

Hey it looks like you got a cap off a Distributor on it instead of one for a Mag.

that is the correct cap, for that magneto. That is how I discovered I had the wrong magneto. The parts stores kept giving me the wrong caps. I then was told to try a cap for a cub and it worked.

That did the trick. It runs better than ever. The last person who installed the magneto probably didn't do it right either, so now it runs great!

I have the correct magneto that doesn't put out much spark. I would like to have it rebuilt. Does anyone know a place to have that done. I called several places around Detroit (where I live) that rebuilt starters/alternators/generators, but none of them rebuild magnetos.

Magman on this site, or Farmallmagnetos dot Com JimN