Husqvarna CT 125 ride-on mower. The blades turn when engaged, but when lowered there is smoke from underneath the mower. The belt is installed ok and appears to be tensioned correctly.

SOURCE: Knocking in mower deck or belt clutch when engaging blades, motor bogs / slows down

YOU MAY HAVE A BENT BLADE OR EXESSEIVE BUILD UP UNDER THE DECK

SOURCE: lots of smoke when I engaged stuck blades - now runs roughly

Most likely what you did was burn one or more spots on the belt. When the blades and belt stop turning, like in tall grass, the engine doesn't stop and will burn flat spots on the belt at the engine pulley. Every time the flat spot hits a pulley, it will jump and can set up a bad vibration. Run your hand around the sides of the belt or take it off and you will feel or see the thin spot(s) on it. Replace belt.

SOURCE: Mower Belt Won't Engage

The fact is that belts don't stretch. They wear down on the sides, causing them to ride deeper in the pulleys and they APPEAR to be longer. Check belt "width" to see if it is less than one half inch. Most craftsman riders use half-inch (AKA: "A section") belts.

Testimonial: "I removed the mower & checked everything - pulleys, bearings, mandrells, springs. All OK. Re-assembled and it works again. Thanks for your feedback."

SOURCE: Craftsman riding mower won't move.

yes it is a bad belt unless it is a hydro-stat(no clutch) you need to look under the mower and see if the belt is on correctly

SOURCE: mtd ride on mower cutter blade belts smoke when

probably a bearing out on one of the pulleys. Disengage turn motor off try spinning each one by hand, you may be able to feel the bad one. You may see discoloration on bad one but probably not with deck on machine.