Please help anyone? I have a Toro 620Z and recently the deck started to shake a lot. Local dealer said my 20hp Onan had a harmonic imbalance. I have replaced all belts but still have so much vibration on the belts that the blade belt keeps coming off.
What model Toro Z is it and what year? If it's a 1998, then there is a pulley retrofit from Toro to keep the mule belt from constantly falling off. Ours was falling off all the time. Also check under the top deck plates where the grass accumulates. This can also cause the belts to work harder and jump off.
check inside of pulley v groove real carefully for anything that might be wedged inside like pebble wood ect... causing it to jump. another way to check is to turn belt by hand and see if belt has any high spots when doing this. that would be indicator of something underneath. maybe belt tesion?
Thanks for all the comments. Toro suggested that I check the pullys and tension on the pullies. I'm not sure about that becase they seem to have a fair amount. I may have wore spots on the pullies from locking the blades up in tall grass.
Loosen the idler pulley till its got flop when you engage it (WITH THE MOWER OFF!). Then tighten it till you get no more flop. I just did it on mine. My belt kept flying off when I let the blades spin down. Also, let the blades of easy by easing the lever back.