Went over on the Red Square Wheel Horse site and found this; PTO keeps disengaging... Hope it can shed a little light on your issue. I don't know too much about that particular style of Wheel Horse , I love Wheel Horse , just not that style ...... Just could never get past the ugliness of those hoods!

 

If it is a model 416-8 or similar this appears to be a link to a parts listing of the components involved in the PTO. Parts – 416-8 Garden Tractor | Toro
Put the diagram and parts list into a pdf - hope it helps.

 

Give it 1 turn IN on the Trunnion on the end of the long rod and see if that helps. The Plate that the Trunnion connects to should go over center and lock in. If 1 turn in doesn't help try 2 turns.

If that doesn't help you might want to pull the PTO pulley and see if there is a chunk of an old belt or something wrapped around the roller bearing race or possibly the needle bearing is coming apart and hanging up.

It is a very simple design and easy to work on but OEM Toro parts can be a bit expensive.

Edit: I meant 1 turn IN, not out lol.

 

I tried turning the trunnion but it's froze on the shaft and I can't see how that would help anyway. When I tried turning it I think it bent the rod a bit because then it wouldn't go over center at all. So I tried turning it back the other way and I guess I bent the rod back, maybe a bit more then it was before, because now it goes over center again. And it seems like it might be engaging a bit better than before because it doesen't seem to disengage as much. It is strange, because sometimes the lever disengages itself very quickly, and usually it doesen't do it at all, and works properly.

 

Adjusting the trunnion to stay over center is the correct way to adjust it. If the lever has some sloppiness to it there is a plastic bushing that goes through the side of the hoodstnd that is probable worn out. You can also put some washers on the lever shaft on the inside of the hoodstand to firm it up.