John D H backfiring - Help Needed

I?m working on a 37-38 John Deere H.
It started running bad and I?m having trouble getting it to run right. At idle it backfires randomly thru the carburetor. If I try to speed up the engine it backfires a little louder thru the carb and engine doesn?t really want to rev up.

For a history of what I already did to this tractor.

When I got the tractor is wasn?t running but it was in fairly good condition. I pulled the head and pistons honed the cylinders and installed new rings. The head was cleaned and valves lapped. Cleaned the points and bought a new coil for the magneto. I happen to have a new carburetor from a 35hp Wisconsin engine, so I made an adapter only for the purpose of testing the JD engine. It really ran good and smooth with the modern carburetor, but once I knew everything was good, I rebuilt and installed the original carburetor. Once I put the original carburetor it still ran good, but sounded more like a JD H should.
Tractor sits in a garage and I would start and run it a few times. About 4 months after I did the ?overhaul? on the engine, we had the flooding from Harvey near Houston. With all the work I had to do, the tractor got left in a building that got about 40inches of water. Before the water got high, I did manage to remove the magneto for safe keeping. Tractor sat in water for about a week. After the flood, I drained the oil, powerwashed everything. Gas tank was completely dry, no water. I took apart the carb and cleaned it very good. I put everything to together, regapped the points, started it up, but now I?m getting the backfiring thru the carb which I never had before. Timing is aligned with the mark and its firing on the right cylinders. I thought it might be the plug wires, so I put some others on it (solid core). Adjusting the carb doesn?t help, so I tried the extra carb. And it was still backfiring. I?ve run it this way for about 30 minutes thinking there may some crude on the valves since water got in the cylinders. Valves are not sticking and compression feels strong. Spark is good. I don?t know whats wrong. The only thing I can think of right now, is I accidently dropped the distributer cap of the magneto on the ground, maybe it has a crack that?s allowing the spark to jump inside. Anyone have any ideas on whats wrong? Any help would be appreciated. Thanks.

Have you tried a new set of spark plugs?

I found they backfire when too lean. BTW, The H came out in 1939.

One that has been in water like that can be a number of things. Dropping the mag cap did not help it any and yes you could have caused a crack which can cause odd problems. Or you have the carb set to lean which will also cause that. Or you may have a valve or 2 sticking which will also cause that

Because of their "lost spark" design (where a spark takes place every time the piston reaches the top of the cylinder) your firing back through the carb & breather is one of three things, an intake valve is sticking open, a spark plug is not sparking under compression but IS sparking when the valves are open or the spark is crossing over in the distributor cap because of a crack.

Very first thing I"d do is as ET questioned... put new spark plugs in it! Usually three heat ranges are specified depending on use. Go with the high heat range!

If new plugs don"t solve it and I had an old distributor cap or another mag, I"d be inclined to swap there next.

Whatever you do, do NOT use a Champion D16 plug! They are simply too cold for what you"re doing with that tractor.

Hope that helps.

Thanks for replies. I forgot to mention that I did buy some new Autolite 386 plugs back when re-ringed the engine. From what I read those are about a mid heat range plug. I pulled the distributor cap and looked at it under a 10X loupe but didn't visually see any cracks. I think I'll clean the plugs very well, in case they are fouled out, I'll recheck the gap and see if it runs any better. I wish I had an extra mag laying around to see what it does.

Thanks, I wasn't sure what year mine is. I need to look up the serial numbers sometimes.

Don't plan on cleaning old plugs to make them fire. I have had brand new plugs that were wet fouled that fired fine outside the cylinder but wouldn't fire under compression. Good luck and let us know what happens.

Lapping valves is a waste of time ,money and effort . Take the head off and do a proper valve job and check the guide wear . Install new valve springs and properly set the valve lash.
Make certain the manifold to head ports are flat and square . Use new gaskets instead of re-using the old gaskets .
Torque everything with a torque wrench.

With 40” of water the engine was under water so the valves probably got a good water soaking. I suspect intake valves or rocker arms are sticking. A half hour run time should have freed them but nonetheless I would pull the valve cover to have a good look at what’s under there.