I got my mower back (2014 Craftsman T1400 42 inch deck ride-on) from repair shop with new Kevlar V-belt (96.5 x 1/2 inch), part 954-04060C and new blades installed by repairman. When the deck is in highest position, it goes full speed but doesn't sound great (clanging in the deck). When the deck is in lowest position, it goes slower speed and no clanging in the deck. Each position I set higher, the blade speed increases and engine goes at higher rpm. I took it back and was told it was fine. It's not. I've had this machine since new. The belt is loose (in my opinion) and I have to overlap to get all blades cut. Neighbour is a handiman and says I need a wider V-belt. All specs say 96.5 x 1/2 inch but I can tell it's slipping and he says get a 6/10 inch V-belt. Is there one? Any chance someone knows the secret? I'm cutting about 3-4 acres. Thanks.
UPDATE: I checked the tightness of the blade belt today. Even with the lever fully engaged there was a lot of looseness in the belt. I grabbed the belt between the deck pully and the drive pully on the mower motor. When it was positioned at fully engaged (cutting grass if it was turned on) I could move the belt sideways 5-6". I bought the belt September 21st 2021 so it was used only about twice ever. It's 96.5 x 0.5 inches and is Kevlar, not rubber. I bought the machine at Sears but they went out of business here.

 

If your machine has a manual blade start arm, then there should be a cable that goes to the deck to tighten the belt for cutting. There should be a spring that attaches to idler pulley arm. When disengaged, the belt should be loose enough that the blades do not spin. When engaged, the belt should be taught. There will be tension differences between different cutting heights, but still should work the same.
Check the heights and levelness of all the pulleys on the deck. They should be on the same plane. I have had more than one mower that I had to experiment to get the right length to work properly. These are usually older machines.

 

Thank you, Freddie. I can change the hydraulics on my farm tractor but can't figure out my ride-on mower! I tightened the cable tonight as you suggested, and it tightened the belt to perfection but when I engage the blades, there's quite a knock from the deck. I checked the blades - tight with zero play. Nothing wrapped around either blade spindle. New blades last fall. Springs are good and tight. I checked the alignment of the pullys and spindles after you mentioned and they're only just fair, I would say. I did notice the deck is lower at front than back so I'll adjust that tomorrow. The blades seem really, really dull. They were the best quality blades, not the cheap ones - are new blades usually pre sharpened or do I need to have them sharpened? Thanks again for your help, Freddie!

 

As far as the knock goes, try and wiggle each blade up and down. There should be little to no play. If so you have a spindle bearing bad. You can replace the whole spindle or just the bearings. They're available on ebay. Some decks have brake pads that touch the pully edge when disengaged. You may have one that is not releasing smoothly.
The deck should be 1\4"-3\8" lower in the front than the rear. This stops the blades from cutting twice.
Blades come pre-sharpened. They do not need a razor sharp edge. You can touch them up with a file or grinder. Less grinding the better. Just enough to get out pits and semi sharp.

 

Okay. I followed your suggestions and it seemed a bit better so I kept using. Both blades were fine with almost zero play at all. All spindles were great. Then... One spindle broke loose from the deck. Hole in the
deck. I got it welded but something's out of alignment. 30 minutes and the belt twisted and came off. Really hot belt. I can get a replacement Craftsman 42" deck but mine has the pto beside the seat and the replacement has pto by the steering column. Not sure how they'll connect together. Thoughts?

 

Both decks have a cable that pulls on the idler pully bracket. Some have a spring on the end of the cable. If the idler pully bracket looks the same, I'm thinking it should work. Make sure all the pulleys are on the same plane and level. If one is higher, or lower, it will heat the belt and cause it to jump off. Make sure all pulleys spin smoothly and quiet and with no wobble. Hot belt, to me, sounds like it's dragging across a surface rather than it gliding across. Check for hot pulleys.