[77newell]

If the pressure at the actuator is changing smoothly but not the rod, it may be that the cylinder/piston interface is restricting movement.

If you have a restriction in the airline between the pedal and cylinder I believe you would see a linear relationship between the pressure at the cylinder and the rod position, but the pressure would rise and drop slowly.

[superbirds]

I think Jon said it earlier, I'd try to connect a pressure gauge in the back to see if the pressure changes when you press the pedal. I planned to fix mine with a rebuild kit, I found the replacement before rebuilding it, plus my old unit did not look great to me. The seal/bellows had a couple holes in it. you can pull that unit off and pull the cover off where the air line connects.

Before I fixed my throttle I drove the Newell 200 miles from Indy to Jenison MI. When I pressed on the pedal it just wouldn't go. Even pressing the pedal down to the floor it took a while for the rpm's to increase. Driving home was an adventure - I got stuck on the side of the road a few times because of the lack of throttle function.
I thought it had to be something in the front pressure unit until after I replaced the rear unit. I couldn't find the front unit to buy so I guess I got lucky. I think the back unit was cheaper and easy to find on Ebay. Throttle works perfect now, which is awesome from where I was at first.
Your's sounds the same to me.
Mine is a 1975 and the old unit looked original to me so I just replaced it as part of my testing process and to prevent future issues.
Good luck - slow an steady always works for me.
Hang in there. Just my experience, hope it helps.

nice pictures - prices seem high tough - http://www.coachconversioncentral.com/airthrottle.htm

[Tf175]

Our 86 also had a tricky throttle when I drove it from Houston,  took a lot of pedal to get going and slow to come back down . I am just getting to that issue now.  Coach has a Williams pedal assembly with a Bendix cruise control /throttle which controls engine throttle lever via a cable . The air cylinder would start moving at 30 psi , then would be reasonably linear up to max travel at 60 
 Psi . Removed air cylinder /cable assembly and added a few drops of air tool oil and now the cylinder rod starts moving at very low pressure . The quick connect at the end of the cable is frozen in every way,  won't pivot . I can source a new cable end  and hardware , but the cable is kinked and needs to be replaced . Wondering if anyone can point me in the right direction for the cable ? Otherwise I imagine I will have to go to the Williams actuator.  The cruise is not that important to me,  but the throttle is aggravating . Sounds like others have similar issues. 
Thanks, Chris

[Little]

Took ours apart couple weeks ago. Lubed it up and it works better than before but still not perfect. Full throttle was about 65 lb at the cylinder but still slow to throttle both up and down. She's in MX at the paint shop so we'll see whats up then.

[Tf175]

We bought a used Williams air throttle actuator off Ebay that was taken off an 8V92 TA . It has bracket , lever , spring , etc. The bolt pattern on the bracket matches the governor cover pattern on the fan side of the engine . This will put the actuator directly under the exhaust pipe , which ,I suppose , will work if I wrap the pipe ( I removed the old , thick asbestos blanket ). Now for my stumbling block : the 86 has an electric /air solenoid to operate the " stop lever " on the governor . This solenoid conflicts with the air throttle bracket and actuator . I believe that the solenoid can be re positioned , but this could get into a time consuming deal . Looking at other positions for the actuator assembly , but the turbo inlet hose is in the way of the lever. I am missing something here , if anyone can shed any light on this I would appreciate it .
I know Kyleb went to the Williams on his 87 , using a cable . Would that be a "push /pull cable ? I would think it has to be as the actuator I have " pushes " when air pressure is applied . Would like to use this unit as it works very smoothly , reaches full extension at 50 psi .
Good day to all , spring is just around the corner !
Chris

[kyleb]

Here are a couple of pictures of what I did. The actuator pushes & a cable works best pulling - so I made a mount that allowed the actuator to pull the cable with the push action.        

I also made a special slider to allow the cruise control to work without binding.

[superbirds]

Nice looking setup - love the platform and cable setup.

[encantotom]

very nicely done.

tom

[Tf175]

Nice set up . The throttle I have doesn't have the lever on the actuator , the rod goes directly to a lever that clamps to the speed control shaft on the governor . Ebay also has the units with the lever assembly you show .Thanks for posting the pictures .
Chris.

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Don’t forget to check for a frayed cable. That was the cure for my 86