RTV900 low torque

[Nluscombe]

Hello folks. I have read and followed the advice of many threads on this subject. My RTV drives OK but has low torque on hills and I dont feel the push of assist motor cutting in even if backing off the throttle. Engine just revs away but power not getting to the wheels - the opposite of bogging down! Its done 2700 hours.

So far I have…
Changed oil (Super UDT) and filters
Checked lift pipes to filter which look OK
Checked and adjusted all linkages best I can, centred servo etc.
Checked regulator assembly which seems to work OK although I had to free up the ‘stop piston’ in the end as it was seized.
Tried to take out the servo piston but couldn’t - guessing is was just connected to the swash plate mechanism.
Changed brass swash plates, mine were a bit scratched and this improved performance a little but it still feels like a car with a badly worn clutch.
Checked pressures, I have 100 on charge and circa 3000 on Fwd/reverse
Inspected assist motor, swashplate and on/off cylinder/spring etc. all look OK
Swapped forward and reverse HST relief valves over.
Loosened bypass valve linkages to make sure they were not open.
I have taken the cover off the bypass valves and they move in and out OK. I was a bit surprise that they leaked fluid (constantly) but I guess this is from a ‘relief’ circuit leading back to the HST housing/sump?

I am considering replacing the HST valves and bypass valves before I give up completely. I dont know much about either valve although the former seem pretty simple.

A new portblock is £550 in the UK which would address these valves, but your advice is very welcome - is it worth it if the pressures are OK - do they fail regularly and would this not show up on the ‘static’ HST pressure tests?

Maybe my motor is just worn and leaky?

Thanks

 

[aurthuritis]

Your pressure sounds good. Have you tried pulling the hand throttle all the way revving the engine full and then feather the peddle and see what happens? Report back

 

[Nluscombe]

Hello Aurthuritis and thanks for your suggestion. I haven’t tried this and don’t seem to have a hand throttle on my machine however I presumed the objective is to build max charge pressure before opening the servo regulator port(s)? I could only do this while stationary by removing the HST linkage and chocking the wheels. I then revved to the max and gently opened the HST lever. This didn’t not seem to make much difference to low revs, and no different to drive afterwards unfortunately.

My brother was here today and suggested we took some HST pressure readings whilst in motion to see if we had a ‘flow’ problem rather than a pressure problem. Also to check we were reaching the 2700psi required to activate the assist pump. This was interesting. We climbed the same short hill in L, M and attempted in H to monitor pressure.
L climbed the hill slowly but consistently reaching about 2600psi and no noticeable push from the assist motor, torque felt weak as usual.
M climbed the hill and slowed down reaching 3100psi and no noticeable push from the assist moto. Again torque felt weak
H failed to climb the hill and reached over 3700psi with no noticeable push from the assist motor.

On the flat in M and H the machine was slow but averaged 2500psi.

I am quite convinced my machine is running solely on the main motor/pump. Any ideas why the assist pump would not seem to add torque?
When I reassembled the motor cover there was slight pressure on the piston spring as I didn’t have the correct hex key to remove this. I am presuming this is normal and I was careful to tighten sequentially to pull in straight.
I am beginning to wonder if it previously had brass plates installed incorrectly. I simply copied positioning of these when installing the new ones.

 

[aurthuritis]

Before you get to far into the hst you should first confirm a few important things. Check your max rpm at no load and with the machine stalled hot oil. Should be 3800 no load 3600 stall. Also check your valve lash and confirm that your engine is firing on all cylinders. And take out the spark arrestor.