I have a 1990 LTF 250 4x4. Starting this year it has began fouling plugs (they are very black and dry, like charcoal). I can get about 20 miles to a plug. Obviously it is running rich. Before I start making carburator adjustments I would like to get some ideas on what may have caused it to be rich. I have replaced the air filter, no impact.
Note: most of my riding is very slow trails, approx. 10 miles per hour.
Any suggestions on how to fix this would be appreciated.

Thanks,
Don