Fork Oil Change

[Hack]

Just swapped the fork oil on my Z900A4.

Manual states oil level should be 426mm from top of tube with the fork fully extended.
Thought I would check the level of the old oil before I removed it as the ride has been very "stiff" and was suprised to see that the level was about 350mm from the top of the tube, i.e. too much oil. This was reinforced by the amount that came out when drained, about 375cc in total for the two legs.

Re-filled as per manual (140cc per leg) replaced caps, leapt up & down on the front end, removed caps and then checked levels, 422mm, near enough for me!

It now seems very "springy" so I'm just wondering why there was so much oil in the first place, was it to compensate for something or have I done something wrong?

All measurements with springs in place.

PS, I used Castrol Synthetic fork oil SAE 10W, dunno why I chose synthetic, is this ok.

Andy

[hanskloss]

Hi Hack, I've done mine with 140-150 Motul medium fork oil each side they might feel a bit springy as you say but cope well with the road, it's not a sports bike anyway.

[Garn 1]

Hack, something seems incorect!
1. 350mm from top.... too stiff. (too much oil)
2. 422mm from top.... too springy. (not enought oil)
3. 425mm from top.... correct via Manual.
4. 375cc for both legs... too much oil (max 350cc)

I wonder if the distance from top of fork leg was supposed to be without spring inserted.
I usually drain and then fill each leg with the stated capacity (capacities did vary between the Z900A4-175cc and Z1- 169cc). I use this method because I have touble measuring, particularly with the spring in-situ. I would also check if the length of the springs are within the service limit 465mm- 455mm
RegardZ

[chrisNI]

The factory manual says that 426mm is the measurement from the top for an A4 okay, but it should need 170-178cc per leg. You measure with the spring in okay, but are you sure you had the front of the bike jacked up enough to be sure that the forks were fully extended?

375cc was a bit too much but 280cc is way too little

[Pigford]

Like all these things... experiment and choose best setting for personal use. The oil will stiffen damping, but springs will still be same... so may risk blowing seals, upsetting handling.. etc!? Try heavier fork oil and just a little more in legs.... But as Hanskloss says... its a 30yr old bike.. don't bother too much as the frame won't be on your side anyway!

[chrisNI]

30 years old or not it will help to have the correct amount of oil in it. A well set-up zed900/1000 handles surprisingly well as long as you don't get over-excited.

[Hack]

Thanks for all the comments.

I think I have now got to the bottom of this:-

When changing the fork oil, i.e. not fully stripping the fork, it is not possible to remove all of the oil as some remains trapped and cannot be drained, hence the two quantities given in the manual.

Oil change only = 140cc per leg
Re-fill after full strip-down = 170 - 178cc per leg

After filling the level should be adjusted to 426mm from top of tube with forks fully extended (front wheel off the ground).
(in my case it came out as 422mm which is very slightly too much oil)

So IMHO anyone who drains the oil and then adds 170cc per leg and does not check the measurement is probably adding way too much oil, that maybe why I found that I had too much oil in mine before draining.

Bottom line is the measurement from the top of the tube is more important than the volume of oil added.

Did 70 miles on the bike yesterday and all seemed well.

Andy

[chrisNI]

To be honest the last time I changed my A1's fork oil they were completely apart. It's easier to measure the level with the leg off the bike anyway because it's easier to get it vertical and the bars aren't in the way, and I think you can get pretty much all the old oil out if you leave them to drain upside down for a while. It's not really that big a job dropping the legs off once you work out a way of jacking it all up which you have to anyway. For the J/R/GPz the level measurements are given for the fork fully compressed with no spring in it so that's really a bench job too.