Tractor Lunging Unexpectedly!! Scary!! Help please!

[heysavoy 19]

I have a 1990 wheel horse classic model # 31-12KE01 with a hydrostatic transmission. I've noticed recently when I put it in reverse, it seems to hesitate to want to go backward. I also found I needed to keep my hand on the gear or it would sometimes abruptly stop. Today I had a scary experience when it lunged unexpectedly to the point the front tires came completely off the ground and almost flipped me over backwards. I took the middle plate off at the gear shift (see picture below) trying to figure out what was wrong and didn't see anything that looked out of order but honestly I don't know what I am even looking for. When I've had other problems, my only saving grace has been this forum which has always been there to help me when I needed it. And I really need some help now because I'm in the middle of leaf season with over an acre of yard and 25 hardwoods. I live in the Charlotte NC area but don't know anyone who works on a wheelhouse especially a 1990. Thanks in advance for any help someone could offer. I love my old tractor and was hoping it would last me another 10 years. 

 

[Ed Kennell 43,165]

Check the cam plate and linkage under the seat.

[WHX?? 53,021]

...then give it a bath and lube job. That kind of dirt build up can wear away pivot points quickly not to mention plugging cooling fins.

Remove the fender pan and clean thoroughly. Inspect & adjust per the manual. 

Cleanliness goes double for hydros. 

[heysavoy 19]

Thanks for the quick reply. Much appreciated. So I assume I have to remove the seat to do so? Told you I didn't have any idea what I was doing? 

Edited 5 hours ago by heysavoy

[gwest_ca-(File Mod) 11,764]

 

[c-series don 10,605]

Yes, the seat and the fender pan in order to give the hydro unit cooling fins a proper cleaning. Compressed air works best. Adjusting the cam plate for the motion control should be in our manuals section. 
 As soon I posted this so did @gwest_ca ! 

Edited 5 hours ago by c-series don

[oliver2-44 10,994]

Go through the manual and note some of the hydro linkage gets lubed and some linkage needs to be dry

[wallfish 18,636]

Could that lurching also be caused by a worn hub key way?

Take a marker or chalk and draw lines on the hubs and axle faces, go in reverse and stop before going forward again. Look at the lines.

Go forward and stop, look at the lines. If the line doesn't stay true the entire time there is an issue with hub(s)

That could explain the reverse hesitation and if it popped up in forward after being in reverse.

[Bill D 2,367]

How many hours on the machine?  If it has high hours the cam and plate under the seat may be worn.  This will cause erratic operation. Had it happen on my GT1800.  Replacing the cam and plate fixed it.  Also check the heim joints on the linkage for play and replace if needed.