1941 JD B, Head Scratcher

Hello Tractor Experts! Bear with me, I am attempting to fill in all the details of what has occurred with this tractor so far, but it has us stumped! My Dad has a 1941 B. Ran like a top. About a year ago, he noticed there was water in the oil, suspected that the head gasket had blown. So, finally got around to removing the head, and replacing the head gasket. By this time, the engine had become stuck too, however, freed up very easily with some lubricant in the cylinder and our homemade flywheel wrench. All parts and pieces are put back together and we attempt to start it. After much flywheel spinning, we OCCASIONALLY (about every 20 spins of the flywheel) get an extremely loud backfire through the muffler, almost like the timing is off. However, we never adjusted anything through the magneto that should affect the timing. We checked that we are getting solid spark through both plugs, and we did pull the carburetor and completely clean it to ensure that everything is getting fuel as it should. We have pulled the valve cover back off in order to view the rocker arm assembly opening and closing the valves, JUST as it should be doing. Does anyone know of anything that we could be missing? Any ideas? Thanks in advance!

Put in new plugs. The old ones are not firing under compression.

Make sure the plug wires are not reversed, and retime your mag according to the book. That should help.

Did you replace the push rod tubes in the cyl. head ? Good chance that is what was leaking ?
As to the backfire like the others said check your plug wires they may of got reversed. I think we all have done that before. LOL

Rings are stuck as well as rust on the valve faces. Hook up a chain or strap and pull it once you've made sure the plug wires are on correctly.

Swap plug wires first.

(quoted from post at 16:09:44 03/23/16) Make sure the plug wires are not reversed, and retime your mag according to the book. That should help.

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Thanks! We had previously reversed the plug wires AND timed it according to the operators manual. Still no change.

Thanks David. We had previously swapped the plug wires, no change unfortunately.

Thanks Mike!!! We actually DID replace the tubes as the old ones looked like hell, it was maybe off of your recommendation that we did so? It was someone on the forum here anyhow! Thanks!