Outdoor Power Equipment and Small Engines - troy bilt tiller plug is fouling and no power

I have a Troy Bilt Roto Tiller (Pony model). The spark plug fouls and the exhaust puts out black smoke. When it starts it runs irradically, smokes, and has little power. I put in a new spark plug but it doesn't help and it fouls after a few minutes. I have tried setting the mixture leaner, etc. but it doesn't seem to help. I have checked the carb to insure it is not loose. I have changed the gas. I have cleaned the linkages, etc. The smoke seems to be more black than wet or oily. Any help would be appreciated.
Thanks, Jerry

I'm not an expert and you should get another opinion but it sounds like your rings are gone by your initial post.

*edit*

To check, I use my thumb on the plug hole but they sell universal fit, cheap compression testers at walmart, autozone, etc

You didn't mention the condition of the air filter. Is it clean? The carb float level could also be too high.
Mike

The black smoke tells me it's too much fuel, so your rings are probably fine. I'd check the air filter, and then move to the carb if necessary.

My money is on the air filter. Don't necessarily assume that since the air filter "looks" clean that it is. For diagnosing purposes, remove the filter and run the engine and see if the trouble goes away. If this yields no improvement, post back with specifics on the engine make, model, specification, and serial numbers.