Could be that the clutch disc got some contaminants on it when it was installed. Any oil or grease on the disc, flywheel or pressure plate can cause a clutch to act like that. I agree if you have changed out rear mounts and still having issues it would be a good reason to pull the transmission and see what is up. Check the flywheel surface and make sure it looks like the surface is smooth, flat and clean. Wipe down with brake cleaner to remove any surface film. Inspect the disc and pressure plate as well. If it isn't looking contaminated then maybe check the brand of those components - if you find Made In China or Hecho En Chine or something like that you might try another brand of components, too. Also check the clutch bearing retainer on the front of the transmission. If it has grooves in the outside surface of the retainer that might not be allowing the clutch to fully engage which is especially important when taking off from a standstill - forward or backward.

Chad

 

 

Make sure your cab mounts are solid and not rusted out.

 

 

I personally don't see anything that looks out of place from this angle. I can see the flywheel, the clutch disc and the pressure plate behind it. But, you wouldn't be able to tell if the disc is contaminated from here most likely. Any oil or grease on the surfaces can quickly get burned into the clutch disc and cause shiny spots that allow the clutch to at least partially slip. If that's not what you think could be happening then regroup and reassess the situation. It's very hard to troubleshoot some of these issues, especially from an armchair sitting hundreds or thousands of miles away.

Chad

 

Pic 1 you can see the clutch disc between the flywheel and pressure plate, as is normal. In pic 2 the clutch does not appear to be there, and is abnormal. From the angle of the pic 2 it kind of looks like the pressure plate hat is not tightened down completely and in contact with the flywheel.

 

Not disagreeing, but I'm guessing that is the angle of that second picture. It's taken looking more "up the front" (non-surfaced) side of the flywheel so the view of the clutch disc itself it mostly obscured compared to the first picture. Just an odd angle for what we are looking for, I *think*.

Chad

 

Good call 455HO. After taking another look I think you are correct. I can see the little wear indicator marks of the disc. Sorry about that.

 

 

A little bit, yes. I imagine if you were to follow the clutch pedal it has a rod that goes down thru the floorboards. And the lower end of the rod should connect to the outboard side of the "Z-bar" which changes the up-down movement of the rod into a forward-backward movement on the inboard end of the "Z-bar". That forward-backward movement goes through a short adjustable linkage to the clutch release fork which pivots inside of the bellhousing and the inside end controls the forward-backward movement of the throwout bearing. The throwout bearing slides back and forth on the bearing retainer attached to the front of the transmission and contacts the pressure plate "forks" of which your truck only has three. Those forks push the pressure plate away from the clutch disc - releasing the pressure it has on the clutch disc which holds it against the constantly spinning (assuming the engine is running) flywheel.

If one of those 3 "fork" tines is broken or there is a malfunction there you could have some issues there, too.

This is just a couple of examples. I'm not suggesting going and buying anything and these are very likely not correct for your truck, but take a look at these links

https://www.napaonline.com/en/p/MBIRCA0632X

https://www.napaonline.com/en/p/MBIRCA5473X

You can see how the first one has three tines or fingers that do the work, whereas the second style has many tines. I prefer this second design myself. More failure points in this case is a good thing.

I hope that helps some. Make sure your clutch is properly adjusted, too. If it is not engaging completely due simply to the adjustment on that inboard end of the "Z-bar" that's the first thing to do. It could be so simple as it needing an adjustment so it can fully engage once again.

Chad

 

Here's a picture out of the 1961 shop manual. Very handy book to have and I don't even own a 1961...lol




Also, if there is no pilot bearing/bushing installed or it is very worn out that could cause bucking, as could worn bearings in the transmission input or output shafts.


Chad