TOPIC:

I have searched quite a few threads about this, and there does not seem to be an agreement on this fluid level/amount. I am wondering if anyone can give me some advice.





Just adding 270 or 290 ml of oil leaves the oil level significantly below the height measurement of 14" springs out forks extended. I am not the only one to notice this.

I use synthetic 5W20 motor oil. I have tried both specs - I have refilled my forks by using just the quantity in ml, and by using just the height measurement. There is a huge difference in how the bike handles between the two methods. I am currently using the height of 14" but am considering draining the fluid out and going back to 270 ml because the front end feels very tight and there is very little compression in normal riding/braking.

I know there is a lot of these bikes on this forum. What is the consensus here?

Edit I would also like to add I am currently running 8 psi of air in the forks. When using the fluid quantity measurement I required about 20 psi air for a firm fork.

As long as the damping rod holes are covered by oil there won't be any difference in damping as a result of variations in oil level. Where oil volume can affect the ride is in terms of how much trapped air there is inside the forks.

A high oil level will leave less room for air, so when the fork compresses there is a higher internal pressure increase compared to a fork with a lower oil level and more trapped air.

Regarding the issue at hand, if you find the forks overly firm then I'd go to a different weight oil. That 5W-20 oil is like 20 weight fork oil (very heavy). Try some 10 weight fork oil and I think you will see a big improvement in ride quality. As for the oil level, I'd follow the manual's recommendation and fill them by measurement to that 356mm level.

I've been using 5w30 to reduce the bouncing, but my forks are worn (the slider ID is inconsistent, and slightly enlarged in the normal range).

I think this has come up a few times in the past, and there was no resolution that I can recall.

According to my interpretation of the hardcopy FSM I have (have not checked agains all of my pdf copies), here's what I come up with:

Model ...... Volume .. Height-Clearance Level
79 500B1 : 220cc : 505mm
80 400J1 : 220cc : 505mm
80 550A1 : 220cc : 505mm

80 550C1 : 290cc : 356mm

81 400J2 : 245cc : 520mm
81 500B3 : 245c : 520mm
81 550A2 : 245c : 520mm
81 550B2 : 245c : 520mm

80 550C2 : 290cc : 356mm

81 550D1 : 244c : 517mm

The Clymer has basically the same info, but lumps the D1 into the 245cc : 520mm group.

Generally speaking, the larger oil volume is accompanied by larger height-clearance measurement (except the C model).
It does seem inconsistent that the C model has such a high volume of oil, but such a short height-clearance measurment.

Possible error in both manuals?
What If the measurement should be 556mm instead of 356mm?

loudhvx wrote: I think this has come up a few times in the past, and there was no resolution that I can recall.

It does seem inconsistent that the C model has such a high volume of oil, but such a short height-clearance measurment.

It has. There are a couple threads about this on this forum, and on other forums. Some suggest to use the height measurement, some suggest to use the fluid volume measurement.

I can offer my experience, perhaps this can be some resolution.

I rode for a week with the specified height measurement of 14''. I was afraid to ride the bike. The front end was very tight and very twitchy and there was almost zero weight transfer when braking with the front brake. The ride was terrible and I was afraid to lean in case a bump in the road upset the now nearly rigid chassis.

I drained the oil and refilled with 300ml of oil. Yes, over the spec but I can measure nearly exactly 300ml. I cannot measure nearly exactly 270 or 290 ml and I wanted the forks to be as equally filled as possible.

After this the bike feels much better. Does not bottom out even with my fat ass on the bike but it is tight enough to be confident to really push in turns. Enough weight transfer under braking to really get on the front brake. In the future I will ignore the FSM 14" height measurement and go by the volume measurement.

Possible error in both manuals?
What If the measurement should be 556mm instead of 356mm?

I feel there is a good possibility this is it. A significant amount of oil needs to be added after the initial fill of 270/290ml to reach the 14'' height measurement.

That sounds like it's much better with the lower level.
Did you happen to measure what the height-level ended up being when you filled with 300ml (300cc) of oil?
You do want both height-level measurements to be the same within a few mm, as the height will determine the air pressure increase as the fork compress.

loudhvx wrote: That sounds like it's much better with the lower level.
Did you happen to measure what the height-level ended up being when you filled with 300ml (300cc) of oil?

I did not. I noticed earlier when filling to the height measurement that each level check would lower the fluid height. I was using a tape measure, and I guess a decent amount of oil was clinging to the tape therefore I did not check the level when I used 300ml because I did not want to change the level. If I decide to use a fluid height spec in the future I will use something else maybe the dipstick from a car. A good example of how a measuring tool should not change what is being measured.

What I can say is I had locked my tape measure at 18" and there was no oil on the tape at 0" with just the initial 270ml fill.

You do want both height-level measurements to be the same within a few mm, as the height will determine the air pressure increase as the fork compress.

Yes that is why I used the quantity I did but that does not account for different amounts of old oil in each fork.

I've been using 5w30 to reduce the bouncing, but my forks are worn (the slider ID is inconsistent, and slightly enlarged in the normal range).

How has this affected the fork rebound??

Heavier fork oil dampens the rebound. In summer I could use heavier oil, but then in fall it would be too stiff.
If you use oil that is too heavy, and hit a series of bumps, the forks, supposedly, will feel like they are pumping down. Most/all of the dampening is on rebound, so if the fork can't recover between successive bumps, it starts to ratchet down. That is in extreme cases, though. I don't think I ever experienced that.

I've tried straight 20w fork-specific oil and it was a little a little thick, especially in the cold (compared to 5w20, and maybe even 5w30).

Actually, looking back at my notes, if they are up to date, I still have two bikes with sae 20w fork oil. I only ride one on nice warm days, and it does seem to be a slightly stiffer ride. It could be imagined, or it could have forks that are less worn out. It gets less miles on it than my other two.

My notes say to try 10w30, but I don't think I have.

So the manual specifies 5w20. Straight 20w feels stiffer, especially cold. And I think I prefer 5w30.
I've been using 20w on two bikes. One feels stiff, and the other with worn out forks seems relatively loose.
The ride I prefer uses 5w30.

However, don't put too much into what I'm posting because all of my forks use much stiffer springs than stock, and they are not all the same. The two running straight 20 use Works spring kits for a 750, and they were not the same as each other. I bought them at different times, and Works must have changed their spring rates. The one running 5w30 uses the stock springs with about 1/3 cut off. That's my preferred spring setup (now that the Works 750 kit has gotten too stiff).

BTW, just to be sure, you are pumping the fork legs a bunch of times after the initial oil filling, to get the air out, correct?

loudhvx wrote: Heavier fork oil dampens the rebound. In summer I could use heavier oil, but then in fall it would be too stiff.
If you use oil that is too heavy, and hit a series of bumps, the forks, supposedly, will feel like they are pumping down. Most/all of the dampening is on rebound, so if the fork can't recover between successive bumps, it starts to ratchet down. That is in extreme cases, though. I don't think I ever experienced that.

I've tried straight 20w fork-specific oil and it was a little a little thick, especially in the cold (compared to 5w20, and maybe even 5w30).

Actually, looking back at my notes, if they are up to date, I still have two bikes with sae 20w fork oil. I only ride one on nice warm days, and it does seem to be a slightly stiffer ride. It could be imagined, or it could have forks that are less worn out. It gets less miles on it than my other two.

My notes say to try 10w30, but I don't think I have.

So the manual specifies 5w20. Straight 20w feels stiffer, especially cold. And I think I prefer 5w30.
I've been using 20w on two bikes. One feels stiff, and the other with worn out forks seems relatively loose.
The ride I prefer uses 5w30.

However, don't put too much into what I'm posting because all of my forks use much stiffer springs than stock, and they are not all the same. The two running straight 20 use Works spring kits for a 750, and they were not the same as each other. I bought them at different times, and Works must have changed their spring rates. The one running 5w30 uses the stock springs with about 1/3 cut off. That's my preferred spring setup (now that the Works 750 kit has gotten too stiff).

BTW, just to be sure, you are pumping the fork legs a bunch of times after the initial oil filling, to get the air out, correct?

Yes I did pump the forks. My neighbors must have thought I was jacking off the bike.

Do you still use air pressure with AM springs or do your GPZ's not require the forks to be pumped up with air?

The reason I ask is I would like to use Progressive Springs and eliminate the air. A friend did this on an older bike and was happy with the result, it is not a KZ however. IMO air forks are poor idea. I guess I need to research this idea some more.

I have not experimented with different weight oil. To this point I have been trying different brands to find something that holds up. 5000 mi on Mobil 1 with new Kawasaki fork seals and the oil was in poor condition. I am now running Nippon 5w20 and will see what happens.

I am considering Nessium's recommendation to try 10 wt fork oil however at this time I don't have any real complaints about ride or handling now that my forks are now longer hydrolocked.

I use no extra air pressure. That is, when the bike is on the center stand, I open the valves to let them equalize to atmospheric pressure. When I come off the center stand, they compress a little. They probably end up with 2 to 5 psi, maybe. But I don't add any compressed air.

I also don't like the idea of too much air pressure. The gpz originally was to have 8 to 12 psi. Once again, maybe it was my imagination, but the forks seemed to have more "sticktion" from the seals holding the extra air pressure. They also tended to start leaking sooner. I've had fewer seal problems with reduced air pressure.

There might be a thread around showing how I cut the stock springs and added a spacer. This gives you a higher rate spring, without having to buy anything, if you already have a propane torch, cutoff wheel, and grinder. It doesn't reduce the travel because there is so much spring to begin with. But it does give a slightly higher initial ride height. (Or maybe it's just restoring what it should have been in the first place.)

I have not rode the bike without air however for kicks I did let out the air in the driveway. The forks compressed by maybe an inch with weight on the front tire. Because of this I will still run air until I can replace the springs.

I assume my springs are original. They still meet the free length spec in the manual but I would not be surprised if they are worn.

A few months ago I got curious about fork oil wights and ran across the chart below on the internet. Two things really stood out to me.
1. The weight printed on the container is more about marketing than about convening information.
2. Some brands loos quite a bit more than others ans the temperature goes up.

From this chart, I made up one of my own based on the actual test numbers. It was pretty enlightening.