I had to make a small wetstone in the shape of the pulley and run the engine while holding the stone in the grove. Then I wrapped some 120 grit onto a rubber block and did the same. Polished the grooves and that solved my problems. I still have all my fingers!

P4- I used a thinner belt, or should I say one that fit the pulley angle better.I also used sandpaper to clean up the pulleys. Not with the motor running... Great weekend for boating in the bay.. 1-2 foot chop perfect

Your belts are slipping.

The number one reason is improper tension, the number two reason is worn pulleys. The number one reason will cause the number two reason.

Belts do not stretch much at all. Actually they will break before they stretch a lot. What happens is that they slip and the sides wear off the belt. This is the black dust you are getting. As they wear they go deeper in the pulleys which gives you the impression that they have stretch or that they are longer.

You probably need to replace your pulleys. In the industrial field that I service you have to replace the pulleys when the belts will not last like yours.

Gates makes the best belt on the market bar none. Put new pulleys on, put the proper Gates size belt on and you should not have to worry with it for several seasons.

Good luck