I'm thinking of giving up on the JD 420

I do believe you are missing a pulley on the deck, the hole in which you have the spring mounted, a idler pulley is suppose to be mounted there. the 60" deck on the 400 i had has 2 idler and one tensioner pulley, here is the belt routing for the 420 60" deck
and the link in which I found this diagram
Service 60 Inch Mower Deck

TCU12447_I9166.jpg
 
Not too much to see on that video.
But , looking at the grease fitting on top of the nearest spindle
In slo mo, I see it wobbling.
Could be the fitting or maybe a bent spindle?
Also, as I watch the belt in the pulley groove,
I see black under the belt, then no black.
That would suggest to me that the pulley is squeezed tight
Then opens up.
I would wire brush the pulley grooves.
Then apply a coat of spray paint.
Run it again.
And see how the paint wears.

Joe
 
Thanks again for all the advice.
The tensioner moves freely, I even got it all greased up when I got he 420 running. I will pull the spring tighter and see what happens. I didn't have any time to work on it today, I put the finishing touches on my 60" finish mower so I can now mow with my Kubota. The yard is kinda looking like tall pasture so I had to get something together to get it mowed off.

The belt diagram picture isn't right, that is for a 7 iron deck, my mower has all the correct pulleys in all the right places. I don't know why JD went with so few pulleys, the belt runs the whole length of the deck (on the backside) with no pulleys to help guide it, that's where it's getting that "whip" effect and kicking off. The closest pulley in the video I posted isn't bent, that gap between the belt and the pulley is because the belt is slacked. If you notice that ripple coming down the belt continues around the pulley and that's the gap that your seeing.

This is the correct belt, if it was too long then it never would have ran for 10 to 15 hours prior to it falling off. The spindles are all straight, I even borrowed a tool for checking end play on engines, I checked out all the spindles and they all seem to be spot on.

I'll check the pulley to see if it's squeezed together, the paint is a good idea!.

I'm really thinking JD has the worse design EVER, I ran my neighbors old Poulan Pro mower for 15 years (mowed 3 acres with it), when I rebuilt the deck on it, all the bearings where shot, spindles were all loose and one spindle was massively bent from hitting a tree stump. That belt stayed on with no issues.

Thanks again for everyone's time
 
Today I hooked a small turn buckle and a wire to the tension spring. I pulled it down real tight and the belt still had a lot of movement. I really thinking it's the belt because it can't be anything else. I took another video, the belt is on correct, the pulleys are inline and not bent.
Check out this video,

 
It's hard to tell from the video but the deck or something is making a lot of noise. To me it sounds like bad bearings in the gearbox or a spindle.
 
I think the camera is picking up more sound then what's really there, the gear box rolls over smooth and it's really tight.
I didn't put two springs on, but I did hook a turn buckle to the spring and I pulled it a lot tighter and it didn't help.
 
Have to go the gear box to tight? They need to be snug with no free play but not tight. Wouldn't effect the belt problem. I still think there is an alignment problem with that pulley the belt jumps off on.
 
A bad spot in the belt will cause the tensioner to jump around. Like the spring is weak. Not convinced that is the problem here though. Easiest way would be get another belt. But that is just throwing money at parts hoping it solves it.
 
Just a thought.... You wouldn't happen to be missing a belt guide somewhere. Either missing or broke off that you're not aware of. I know it might be dangerous but how about holding a hammer handle or something similar against the belt and see how it seems.

Disclaimer: I know the safety police are watching so if you don't like the handle thing DON'T do it.....
 
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