Fork Clunk at Extension RD400E

[Hemi]

Thanks Jspooner!
That gives me something to work with.
Found a couple parts diagrams.
Looks like there are several parts that come together in the  bottom of the forks.
Perhaps the previous owner re-installed incorrectly.
Hemi

[Hemi]

Jspooner,
I've checked several parts diagrams and I don't see the springs you noted at the bottom of the forks?
I can't seem to paste a photo here.
Hemi

[1976RD400C]

Jspooner,
I've checked several parts diagrams and I don't see the springs you noted at the bottom of the forks?
I can't seem to paste a photo here.
Hemi

I took that picture this morning of the fork pieces. I don't think they were ever available?? Yamaha had the fork tube come as an assembly with them installed.

[Dvsrd]

There are several possible contributing causes to this "top out clunk" phenomenon:

- Too much spring preload and/or too stiff spring rate
- Not enough rebound damping (Oil too thin, or someone may have modified or messed with the damping rods/ valving)
  There should normally be significantly more damping on the rebound than in compression. Just remove the spring, add
  a little more oil, and test by hand.
- Excessive unsprung weight. Like a heavy aftermarket fender, a second brake disc and caliper.I had this "clunking"
  during accelleration on my then totally stock "Euro" XS650, with extremely heavy OEM dual front discs and a heavy
  OEM chrome front fender. After upgrading to a much lighter brake setup, etc, I lost around 10 lbs of unsprung
  weight, and the "clunk" was gone.
- Some bike also have a "top out spring" that will give the forks a "soft stop" as they extend fully. These are often
  20-30 mm long, and sit on the outside of the damper rods. Just google "top out springs".

[m in sc]

the top out springs aren't in the 400 forks... but they are in the xs650 ones. just as a point of interest.

[Hemi]

Thanks Dvsrd,
I've tried more fork oil, thicker oil, less oil.
With more oil, the forks seemed a bit more extended at rest, felt stiffer, less sag, still had the clunk.
I'll check the other things you mentioned.
Hemi

[Jspooner]

Thanks Dvsrd,
I've tried more fork oil, thicker oil, less oil.
With more oil, the forks seemed a bit more extended at rest, felt stiffer, less sag, still had the clunk.
I'll check the other things you mentioned.
Hemi

I'm still going to say its the "top out valve" as 1976RD400C calls it. As I said, you can assemble the parts MORE than one way, but it only works ONE way.

[Hemi]

Jspooner,
Appreciate your advice. Seems you've experienced the very same issue and found a fix.
Fingers crossed that's the issue.
Sounds like it took you a few attempts to get the parts inserted correctly.
Hemi

[Dvsrd]

the top out springs aren't in the 400 forks... but they are in the xs650 ones. just as a point of interest.

Mark, yes the later years of XS650 had top out springs, but my 77 XS650D does not. It is really a bit weird that the rather wheelie prone RDs did not have them :)

[RDnuTZ]

so, is there any consensus on the forks with top out springs being preferred over those without? If so, maybe an easy bolt on improvement?

[m in sc]

IMHO it doesn't matter. i've never had this issue unless the preload spacer was too long or the wrong parts were in there. somebody did some 'creative' shit to the 70 R5 and one was way longer than the other and used to do this. stuffed regular RD internals in it and swapped the tubes and haven't had an issue since. It's not a common problem on rds unless somebody was mucking about in there.

[Hemi]

Jspooner,
You nailed it!
Disassembled the forks today, and found the top out valve had been reassembled incorrectly by the previous owner.
The oil flow ring that contains the spring was upside down, easy to see how that could happen.
No more clunk at extension, hard to believe that little spring and ring make such a big difference.
Many thanks!
Hemi

[Jspooner]

Jspooner,
You nailed it!
Disassembled the forks today, and found the top out valve had been reassembled incorrectly by the previous owner.
The oil flow ring that contains the spring was upside down, easy to see how that could happen.
No more clunk at extension, hard to believe that little spring and ring make such a big difference.
Many thanks!
Hemi

Glad you figured it out. I remember thinking the same thing when I figured it out.