Outdoor Power Equipment and Small Engines - Murray riding mower issue

I own a Murray lawn tractor model #46581x92a and need to adjust my primary belt deck tension. The mower calls for part # 37x89 which should be 87 inches long but it is still too loose. I've purchased an 86 inch which is sligthly loose but an 85 inch is too small. I suppose i can shop for a 85 1/2 inch but i should be able to make the 86 inch work if their is an adjustment somewhere. The mower cuts fine until i hit thick grass. The middle blade stops cutting. I've changed the secondary belt and it fits fine. When the pto handle is engaged if i spin the pulley with the secondary belt it drives all the pulleys but when i spin the pulley on the opposite side that has the primary belt on it the pulley will turn but the belt is not tight enough to drive the deck. Please help

If it doesn't work with the proper belt, there is a problem that needs attention. Is the tensioner not moving? I mean, 1/2 inch should make no difference when the tensioner pulley moves several inches to take up any slack.

I don't know what you mean by the tensioner. I know that once the pto handle is engaged it tightens up quite a bit, but another 1/2 inch would do the trick. I thought maybe there was some type of adjustment that I could apply to the pulleys like the alternator belt in a car.

I fear that you have threaded the belt incorrectly. Check your manual to see the proper routing of the belt. Sometimes it is on a sticker on the deck.

Are you using the OEM belt,, or a generic aftermarket??? Lots of companys have belts that are specific sizes to there equipment & the aftermarket only has sizes in inches.. Also the way the belts are measured,, Some are measured using I.D, & some use O.D... & also the pitch of the "V" part.... I always try to use the OEM as it prevents headaches like this.... Roger