I recently acquired a 75 DT400B that I proceeded to completely tear apart and rebuild from the ground up. On the first few test rides, I did a couple good throttle twists and thought the rear tire was spinning. No notice of tracks in the gravel so I added a bit more freeplay to the clutch adjuster at the lever. What I found was that when accelerating the freeplay of the cable would disappear and the clutch would slip. I ended up adding half travel of the clutch lever as freeplay and still having the freeplay removed under acceleration, whether it was a free rev or under load. I have checked everything that I know of and have not found anything that stands out as being able to do this.

I pulled the clutch cover and tried to see if there was excessive travel in the clutch basket like a missing thrust washer or something, but it has very little movement. Nothing that would cause what I am experiencing.

As a bit of history, the engine was completely disassembled, cleaned, bored, and checked for any issues. I replaced all the bearings, seals and gaskets along with new OEM clutch fibers and steels with new springs. I have a neighbor that was a 35 year long motorcycle mechanic, so he has helped with most of the engine/gearbox process and did the final checks and assembly of the engine.

We are both at a loss for a solution so any ideas, comments, questions, complaints, criticisms are greatly appreciated.
Bryan

When you checked with the cover off did you remove the pressure plate? 

It sounds like the outer basket is getting forced towards the engine under power? (which the helical gear might do) You might not be able to feel/see the issue with the pressure plate installed and double-checking the shaft nut would be a good idea.

Yes, I did remove the pressure plate and all the disks. I believe there was only a few thousands movement. Nowhere near what would be needed to take up that much slack. Everything inside this was all torqued to specs according to the shop manual.

The two main possibilities that come to mind are that something is not assembled correctly in the clutch/engine or what you're seeing with the cable is due to a routing issue that somehow pulls the cable when accelerating?

Posting photos (Imgur, Photobucket, etc.) or better yet, video (YouTube, Vimeo, etc.) would help someone here to help you figure it out.  We can't see or experience what you're seeing.

I would like to do that, but at this time the engine is out of frame and completely disassembled again to try and find the issue. I am going over it piece by piece according to the exploded parts diagram to see if something is missing or out of place. If I get it back together and it still happens, I will definitely take a video of what is happening.
I do appreciate all the input and help so far.

Be aware that Yamaha parts lists and diagrams are NOT "assembly diagrams" and often contain errors or misleading information.

I can't clearly remember the clutch stack on that model but memories get jogged when seeing pictures of, say the thrust washers and spacer order.

I'll stick my neck out after reading the first part of the story--i'm tired & past sleep time.
My guess is that if bearing #13 has a circlip--then it's missing.
Guess two is if circlip is there then plate #14 is missing.
I had a bent 'say bowed' plate on my early 360 once & had movement on that mainshaft so had your problem but in a very minor annoying way as the helical clutch & crank gears pulled it sideways & caused clutch slip unless i had extra lever clearance--until i fixed it.
www.partzilla.com/catalog/yamaha/motorcy...ission-250b-c-400b-c


 

I went over the disassembly and did note that both of the pieces you had mentioned are there and it appears they were in the proper order. Although the screws for the plate are a bit hammered, they are tight.

Add an extra steel plate to the clutch pack and adjust it . The old clutches get worn down and the pressure plate bottoms out and slips
Doug

Doug, as stated in the first post the clutches were new OEM plates and steels. New springs were also installed. I'm hoping they're not worn out or burnt yet.