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I have a tiger cub 48" ztr. The drive belt came off once...put it back on was fine. A few mows later it started coming off at start up. Belt looked worn so I replaced it.
Still kept popping off at either throttle up or when putting it in "drive." I could put it back on once or twice and it would be good and stay on.
Started becoming more frequent so I replaced what looked like a worn idler pulley. It seemed to be slipping off from this pulley.

Put new pulley on and it's more frequent. Today I put the belt back on 15x and gave up. It would pop off shortly after start up. If I was low throttle or high throttle didn't matter.

I was able to see it slip off the idler pulley which is brand new.

Pulleys all look aligned to me. I give up.
Any help greatly appreciated. Thanks

It's sliding down part #9...in photo.

Number 11 is a double pulley? Make sure the belt is on the upper pulley.

BlazNT said:

Number 11 is a double pulley? Make sure the belt is on the upper pulley.

Click to expand...

it look like a single pulley to me. Here's a link to that part number https://www.amazon.com/Stens-275-945-Heavy-Duty-Pulley-482744/dp/B001OK8KI4
thanks

You may be correct. Model number of mower so I can check more. If it is just a single v pulley then your idler pulley bracket is bent and needs to be straightened. Take a straight edge and lay it across the 2 v pulleys number 11 and 19 and see if idler pulley in lower of higher than it should be.

Model/serial - STC48V- KAI serial: C6001027
Adding additional photo - complete parts page

Check that part 5 is tight and has part 4 installed correctly. Also check part 3 to see if it is working correctly. It could need changing.

I could not find your model number. This is what I have to choose from.
SCZ48V-22FX - Scag Cheetah Zero-Turn Mower, 48" Deck, 22hp Kawasaki (SN: J9600001-J9699999)
SCZ48V-23CV - Scag Cheetah Zero-Turn Mower, 48" Deck, 23hp Kohler (SN: J9700001-J9799999)
SCZ48V-25CV - Scag Cheetah Zero-Turn Mower, 48" Deck, 25hp Kohler (SN: H6800001-H6899999)
SCZ48V-26BS - Scag Cheetah Zero-Turn Mower, 48" Deck, 26hp Briggs & Stratton (SN: K4400001-K4499999)
SCZ48V-28BS - Scag Cheetah Zero-Turn Mower, 48" Deck, 28hp Briggs & Stratton (SN: H6900001-H6999999)
SCZ48V-691FX - Scag Cheetah Zero-Turn Mower, 48" Deck, Kawasaki 691FX (SN: H6700001-H6799999)

Check the tensioner and idler pulleys as they have a bearing in them.
Check the tensioner arm bearings, #15. If the tensioner arm twists due to a bearing problem, then the tensioner pulley will not run true.
Both pumps have a single groove pulley, you probably have stamped metal pump pulleys. The parts diagram shows a cast iron pulley which is now the replacement for the stamped pulleys. Check all the bolts for tightness as a loose bolt on one pulley will cause the pulley to not run true.

BlazNT said:

Check that part 5 is tight and has part 4 installed correctly. Also check part 3 to see if it is working correctly. It could need changing.

Click to expand...

5 is tight, 4 seems like it can only be installed 1 way it's just like a symmetrical sleeve. Unless i missed something on it. The spring seems to be pulling tight #3.

Mad Mackie said:

Check the tensioner and idler pulleys as they have a bearing in them.
Check the tensioner arm bearings, #15. If the tensioner arm twists due to a bearing problem, then the tensioner pulley will not run true.
Both pumps have a single groove pulley, you probably have stamped metal pump pulleys. The parts diagram shows a cast iron pulley which is now the replacement for the stamped pulleys. Check all the bolts for tightness as a loose bolt on one pulley will cause the pulley to not run true.

Click to expand...

I'm thinking its #15 now. The idler arm has quite a bit of play in it, and i noticed it's also got play up and down in it...not just a little play from the spring back and forth.
I'll replace the bearings in it and see if that helps. Should be quick, cheap, & easy to find local part.
Thanks