What makes a gas engine run rich

What plugs are you using now, and which have you tried in the past? Have you removed a plug and looked at the spark to see if it's weak?

Have you tried to operate it without the air cleaner hose attached? Is the choke plate operating properly; especially, is it opening all the way? Does the air intake to the carb. look wet. If so, under what conditions? Does the tractor appear to operate differently with the air cleaner connected (if so, it may be partially plugged - not a bad idea to pull it off, fill it with kerosene or diesel to clean its screen, and drain it good before reinstalling it in any case - supposed to be done periodically, but probably hasn't been since the tractor was new).

How far off their respective seats are your adjustment screws on the carb. set? Have you REALLY tried to move these as close to closed as the tractor will tolerate and still run? These control the fuel/air ratio, and are the most "obvious" source of too rich a mixture.

Have you looked for leakage around the intake manifold? Unlikely, but a leak here could necessitate overly rich adjustment at the carb. and result in fouling at the plugs.

Have you checked the timing and the valve lash lately?

Might be most anything on this list, which hopefully is helpful - let us know how you make out.

In your comments below, you indicate that your carb. doesn't seem to have a main fuel adjustment, just an anti-dieseling solenoid. Too much fuel flow has got to be your prime suspect since you're getting black smoke, strong gas smell, etc. There has to be some sort of restriction on the incoming fuel other than the solenoid valve. Most IH carburetors have a main screw - do you have an operator's manual for this tractor? If so, it should have a section about adjusting the carb. If not, buying a manual would surely seem like a low-cost option to all of the other things that you've done. Chances are about 99 percent plus that your problem is in your fuel supply control somewhere.

At this point, I'd suggest you again try operating for a while without the air cleaner hooked up to the carb. - too little air flow through it is about the only thing other than fuel supply control which would cause the severity of the problems which you describe. If the problems don't magically go away without the air cleaner hooked up, the problem is almost certainly in the fuel supply control.

Good luck, and let us know how you progress.

The float needs to be cut down on fuel. Reclean the carb and readjust the float to a smaller than factory spec. When you reclean the air passages, use Berryman B-12, use a small copper wire to run into the passages. A strand from multi-strand copper wire works well. If any improvement then keep tearing down the carb until clean. I did mine three times and each time I swore it was clean the time before.

T_Bone

Thanks for the help and ideas. I have a manual for that tractor (just operators manual), and it states how to adjust the idle adjusting screw. But, the fuel shut off solenoid "permits full fuel flow through the main jet when energized". The "full fuel flow is stopped when the solenoid is de-energized". And that is it as far as adjustments to the carb is concerned. Now, the main jet, that is part of the solenoid does have two different size orifices listed in the parts manual. And you are right in that it may be the wrong size orifice. I will order the one that is supposed to work for this carb on Monday and see if it is the same size. I bought this tractor when it was hardly used at all, and it ran fine for the first few years. It is possible that someone could have changed jets on the solenoid when I had the carb worked on years ago. I will change the float level to allow less fuel in the reservoir as you suggested. There is one peculiarity that I haven't mentioned. This tractor does have a fuel pump. (I changed it too!). When the tractor is shut down after using it, everything is fine. When I go back several hours later, there is a tiny bit of fuel seeping from the air intake side of the carb. This continues to seep when the tractor is not being used. When I start the tractor up and start using it, this fuel seep dries up after a few minutes of operation. Not sure what thatt indicates, but thought I would mention it. I thought that the housing might be cracked on the carb, but if it is, it is invisible to the naked eye. Anyway, if persistence pays, I should find it someday. Thanks for your help !!!!

This may be a good bit of your problem. Try setting the float so the level in the bowl is lower.

Good luck, and let us know how you progress. If you still have the strong gas smell, be careful what you're doing so you don't end up with a fire.

What does your exhaust look like?
Are you using oil?
How's the compression in the engine?

Repeated question:

How far off their seats are the adjustment screws on the carb.?

Like I indicated before, could possibly be a leak in the intake manifold somewhere but this is really not very likely.