I have a craftsman riding lawn mover 42" model #917270812

Either belt is not on right or one of the tension springs needs to be adjusted or it has come loose...

there is no clutch as far as I know on ride on mowers, there is just a tensioner pulley that works off the brake/clutch, to adjust it you have to depress the brake/clutch to take the tension off the belt then adjust the belt tensioner to apply more pressure on the belt. Also check the tensioner pulley as I have found that they split in two and the belt becomes loose as a result.

Hope this helps

just loosen the adjuster and when it loose pull-out the loosen belt and replace with the new one and retight the adjuster at exact belt tension

Things you need to check, loose blades, blades out off balance, bad belt or loose belt, bearings in the blade mandrels worn.

There is no adjustment on the Craftsman mower belt tension. The tension is maintained by a spring. If the belt is to loose you either have the wrong belt or something is broken like a mandrel housing that holds the fixed pulleys in place.

look under unit and check the pulleys may need an adjustment on the engagement pulley which is connected to the foot brake pedal if adjustment good than also check rear pulley unbolt rear cover to access sometimes bolt becomes loose if good and belt is damaged or stretched than replace belt

the lever moves an arm on the deck connected to a pivoting plate holding the idler pulley. when you push up the lever, it pulls the pivot [and idler pulley] back and puts tension on the belt. if this pivot gets rusted, it sticks and won't release tension on the belt. find the pivot point and lubricate it thoroughly with oil, wd-40, or liquid wrench, then work it back and forth by hand until it is freed up

Check proper belt routing. These models are a little tricky. Also check tensioner spring.

You can go to the Sears website an download a free owners manual that shows what you need.

It sounds to me like a very loose belt or very worn out belt and also an adjustment problem.