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Chris Huber
Chris Huber, Small Engine Technician
Category: Small Engine
Satisfied Customers: 625
Experience:  20 years experience. Briggs and Stratton Master Service Technician. Dealer for most engine lines.
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I have a Murray 12.5 HP 38 B&S engine riding lawn mower and

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I have a Murray 12.5 HP 38" B&S engine riding lawn mower and recently broke the drive belt. I purchased a new one which worked for a couple of days before the belt jumped of the series of 4 pulleys. I put the belt back on the same way again per the belt instructions. The up and down lever will not start the blades to spin when you push the lever from the "stop" position to the "start" position. The mower was manuf. in about 1989. I do not know how to adjust the tension on the belt. I do not have a manual. If this is a tension adjustment problem can you explain how to adjust it or is there another problem. My new belt already looks pretty ragged on the outside edges only after a couple of hours mowing time. Thanks Mickey Smith in Huntsville, e-mail is "[email protected]
Murray decks typicall run deep v pullies and thin 1/2 belts, so the belt penetrates the pulley well. thus making the belt stay on. Take the blades and see if you can move them up and down from the tip to check for a worn mandrel bearing (spindle). Also check the idler pullies and the tensioner pulley as well for wobble. If anything is not spinning perfectly, it can cause the belt to come off. Under the seat should be a model number. If you give me the model, I should be able to post a parts breakdown for you.
Customer: replied 8 years ago.
my model # XXXXX XXXXX MFG on 98107 listed under the seat ........Under the engine cover it lists for the38 inch blade "Motion Drive Belt 37 X 87" and Blade Drive Belt 37X 86" Not sure why two different numbers for only one belt. My first belt from WalMart was a # XXXXX ?? which was about 12 inches too long and the belt diiagram showed two tensioner pulleys. I exchanged the belt for a 40" belt ( 37 X 62) which fit all the pulleys fine. The mower ran fine in both the on and off cutting posistions for an hour or so until the belt suddenly came off the pulleys............... the belt was pretty much frayed along the whole side edges on the widest part of the belt. Did I purchase the wrong number belt as indicated about ? Walmart only had two different sizes.....the longer belt was $ 26 and the shorter one (which I bought was $28 for some reason) Do I need to fine a Murray or Briggs & Stratton dealer for the specific belt number ? Thanks Mickey in Huntsville, Al.
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