The answer comes from a frequently appearing repost:
Hydros (internally at least) are considered to be one of the most robust components of a CC...they just NEVER wear out. They do start to develop "hydro speed creep" over time. This is when the machine speeds up when descending hills (without moving the dash lever) and slows down when pulling heavy loads or climbing hills (again without moving the lever). If your machine does this see below. If it wants to "auto accelerate" (dash lever moves on it's own, without you touching it) then you need to tighten the dash lever friction (see manual for proper adjustment procedure)
Hydro Speed Creep
Your problem is not related to the hydro itself, but rather to the linkage that connects the lever to the hydro.
Before you go any further, order a service manual for your 1250 from one of our sponsors (see ads at www.ihcubcadet.com). It will be very valuable in working on your machine and will explain some of the adjustments required after your hydro fix. They are not very expensive.
1st step: Remove the frame cover between the dash and fenders, this will expose the hydro, it's linkage, and the drive shaft.
Take a good look around, you will see the hydro unit and some goofy looking linkages on the left side of it (from drivers seat). This linkage is your problem.
There should be a triangular (mostly) plate standing on edge with a slot cut in it. This should be connected to a bent arm that goes into the hydro. The two pieces are connected by a coil spring with steel button end caps. The spring/slot is where your problem is.
The function of this goofy linkage is to act as a shock absorber for the hydro input. The bent bracket that is attach to the shaft that goes into the left side of the hydro is what moves the swash plate inside the hydro, causing it to move you fwd/rev. It cannot be directly linked to the lever on the dash because the system would be very "jumpy" and would buck you off the seat with every speed change. The spring serves as a shock absorber to dampen the linkage between the lever and the hydro, smoothing out starts and speed changes.
Problem is that with time and normal wear the spring gets weak and the slot wears to a longer than original length. This "softens" the shock absorber effect of the spring and allows the hydro to take the path of least resistance and compress the spring (changing the tractor speed) when the drive load changes without a corresponding movement of the lever on the dash.
Fix is fairly simple, but will require the manual I mentioned above. Remove the hydro unit, either by removing the fenders or by pulling the rear end. Disassemble the linkage. Add material to the worn slot with welder and then reshape to original dimensions with file/die grinder. Install new springs and steel end caps. Put it all back together and make neutral adjustments as shown in the manual.
This is also a good time to replace the hydro to tranny gasket (a common leak point) and install a new cooling fan if yours is damaged.
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