What would cause Hydro Transmission to bind?

[Turbodriven]

The mower is a Gravely 260z with (let's just call it) a brand new engine/fuel system and for the sake of argument, a new deck/blades/pto. Engaging blades causes a momentary drop of a 4-500rpms but then quickly high idles again at 3400+. About 10-20 feet into mowing however (sticks forward) the hydro system binds and causes the motor to lose significant rpms (tiny tach hooked up to watch rpms). From an operating speed of 3400+ down to 1700 or even less depending on load. Going up a hill for example causes rpms to go so low it almost kills the motor. Pulling the sticks back to neutral at any point causes the rpms go back up to 3400+ (within a few seconds).

1. When on jack stands, operating the left stick full forward has no effect on rpms. Wheel spins like it should and literally the rpms don't even dip 1 rpm. Same with the right stick.
2. When mowing, operating the left stick (only) full forward from a stop causes rpms to drop to well under 2000. Same with right stick (only). No difference between the left and right. Both independently cause the rpms to drop significantly.
3. With belts off, both pumps feel smooth (hand spinning the pulleys). Maybe that says nothing but it's worth noting.

What could be wrong in the system pictured to cause this? Blockage on the pressure side? But on both sides? Seems highly unlikely that two independent systems would have pressure side blockages? The filter is on the suction side and shouldn't cause something like this right? And even if it was dirty/blocked it has a bypass to let fluid go through unfiltered.

Note, the system is filled correctly with 'what feels like' clean fluid which is the same consistency as the new bottle of manuf. recommended fluid I have. Nothing abnormal. It also has a brand new drive belt.

 

[ILENGINE]

clogged inlet side of pumps would cause cavitation and lose of power to the drives, not lose of engine rpm. I can't see having a problem on both drives at once. I have seen a similar issue with a drive motor that was binding when it got hot. But not both sides at once, unless both wheel motors are going back at one. little chance. I would be checking to make sure you are not loosing spark or otherwise dropping a cylinder on the engine causing loose of power.

 

[mechanic mark]

RPMs DROP ON GRAVELY 260Z WHEN MOWING - Google Search

www.google.com

Replace spark plugs, Clean underside of deck after each mowing.

 

[Turbodriven]

clogged inlet side of pumps would cause cavitation and lose of power to the drives, not lose of engine rpm. I can't see having a problem on both drives at once. I have seen a similar issue with a drive motor that was binding when it got hot. But not both sides at once, unless both wheel motors are going back at one. little chance. I would be checking to make sure you are not loosing spark or otherwise dropping a cylinder on the engine causing loose of power.

Yeah, I've been down that long road. Freshly rebuilt motor. Brand new MDI ignition system with new plugs. Completely new fuel system including carbs (which even new, were opened up and gone through). The motor low and high idles great. Runs smooth through the full rpm range sitting still. Just falls on it's face when under hydro load.

 

[StarTech]

Also drive belt idler pulley and idler arm. The pulley bearing maybe going bad.

 

[Turbodriven]

Also drive belt idler pulley and idler arm. The pulley bearing maybe going bad.

This spins free. But if it didn't, this would present itself even at idle. Not just under load.

 

[bertsmobile1]

Your symptoms are consistent with an engine running on one cylinder
What sort of drop off do you get when engaging the blades ?
Running too heavy an oil in the hydro can do similar
I once tried HD 40w 70 in a trans to see if I could squeeze a few more seasons out of it
Works fine to about 2/3 pedal movement then too much resistance on the pump pistons