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I have an RZT50 and have had no power problems with transmission. However, on the last mow I noticed mid-way through that a heavy vibration was occurring all the sudden. I killed the PTO thinking it was a deck problem of some sort and it continued. After inspection, I noticed that the transmission pulley was bouncing or vibrating. I released the Idler Bracket Spring to make sure it wasn't the main shaft and the vibration and bounce stopped. With the mower running, I carefully pulled the spring by hand to increase tension on the transmission drive belt and as it sped up the vibration would start. It's almost like the tension pulley is trying to bounce on and off of the belt as it goes around. Could this just be a drive belt that needs replaced (stretched)? Or possibly the idler pulley itself (bad bearing or something keeping it from spinning freely) The belt does not look worn, but I noticed that the idler pulley was nearly fully extended when the spring is re-engaged. I turned the two transmission pulleys by hand and they seemed to spin fairly smoothly. Any guesses on first steps? Thanks for all the posts.

Hello Brian & WELCOME to LMF...
Take a closer look at the belt... It sounds like it may have a part of the inside missing... :smile:KennyV

I did notice that the belt had some sort of bump in it, but I could not see that it was what was causing the jumping. I might try replacing the pulley and the belt both. I was hoping to avoid the cost of one though if I could. I will inspect the belt again to see if there is a chunk or anything missing from the inside. Thanks for the suggestion.

If the pulley looks good and turns free, there is no need to replace it... :smile:KennyV

Hi Brianholland


Welcome aboard to LMF hope you enjoy this place as much as i do, there are very smart people on here to advise us all the possible lawn mower problems. Yes Kenny is right it's your belt there is some missing on the inside of your belt i have one with the same problem. What i did to mine is i smoked the belt in one spot and burned the belt on that spot and every time it comes around to the idler pulley it jumps over that burned spot to make it vibrate and makes the clutch paddle move back and forth to. I don't think it's your idler pulley, i would try the new belt first too i bet that solved the problem:.

Alright, that's two confirmations on the belt so that's the direction I am going. I will post back with the results. Thanks for the great forum.

Just a follow-up in case others have a similar problem. While taking the old drive belt off I was inspecting the right hand transmission pulley. While spinning it around I noticed something wedged into the grove of the pulley. It was something that looked like wood but had somehow melted to the pulley itself. A quick pop with a flat head screwdriver removed it and the mower is running like new again. Picture is attached. Do a careful check of each pulley if you have a similar problem.

brianholland said:

Just a follow-up in case others have a similar problem. While taking the old drive belt off I was inspecting the right hand transmission pulley. While spinning it around I noticed something wedged into the grove of the pulley. It was something that looked like wood but had somehow melted to the pulley itself. A quick pop with a flat head screwdriver removed it and the mower is running like new again. Picture is attached. Do a careful check of each pulley if you have a similar problem.

Click to expand...

This website is FANTASTIC! My Troy-Bilt RZT50 developed a sudden vibration during mowing. Like many here I figured it was a bad drive belt. I read this post and before I took everything apart to replace the belt I checked the pulleys on the hydra-stats. Sure enough there was a piece of pine cone wedged in one of the pulleys. Plucked it out, problem solved!! Thanks everyone!!

brianholland said:

Just a follow-up in case others have a similar problem. While taking the old drive belt off I was inspecting the right hand transmission pulley. While spinning it around I noticed something wedged into the grove of the pulley. It was something that looked like wood but had somehow melted to the pulley itself. A quick pop with a flat head screwdriver removed it and the mower is running like new again. Picture is attached. Do a careful check of each pulley if you have a similar problem.

Click to expand...

Came here looking for help after removing the drive idler pulley with no success. I saw these posts and reinspected the remaining pulleys to find a pine cone as well. Running well again. Thanks!