Hello K100 owners, enthusiasts, and Gurus...

Hoping for some tips on my front forks. They were feeling hoppy especially in higher speed corners which is a little disconcerting at the least! No damping, thus no rebound. I decided to change the oil. I started by placing it on the center stand and elevating the front wheel, I drained the fairly clean oil by removing the 10mm bolt at the bottom and then the allen bolt from the top. I re-filled with Spectro 5W fork oil to 360cc. No difference at all, the fork is actually resting at bottom, I have to pull up to get any action?

Please help!

Jason

Hi Jason.
I would have said that 5w is a bit on the light side and would use 10w.
K100's are notoriously fussy about how much oil goes in each fork leg, too. Last one I had was seriously firm and uncomfortable to use on bumpy roads. Too much oil.
Replaced it with 365cc(?) Of 10w and it was great.
The other thing to check is to see if they are the original forks. As yours is an '85 they may have been changed at some point and there are a few different types.
You'll find a number cast on the side towards the wheel of the alloy sliders that can help to identify them.
Only other thing is if it has big miles, many do, it may be time to change to a pair of progressive springs.
Do a search on the forum about forks. It's a bit like opinions on tyres, plenty of information.

Hi Jason, new oil wouldnt change problematic handling of old K specially in curves.I agree with Comberjohn and  reccomend to think about progressive springs for your old forks. I changed old tired springs in 83 forks with Wurth progressive springs an fill with 400 ml of 7,5 w oil. It changed a bike handling totally.
Best regards

Sounds to me like the springs are sagging a lot and by the picture you posted they are sagging.

Thanks gents... Time to pull it apart. I am in Canada and just did an extensive Google search for new springs. I could not find any! Any suggestions for sourcing?


 Anyone familiar with the alloy pieces under the plastic cap with the oil fill allen bolt? You need to hold them with a wrench in order to loosen and tighten the allen bolt. why is it made to turn? is it some kind of preload adjuster?

That is a bung with an O-ring and is free floating, held in with a wire circlip.  You push it down and get a fine point and pick the clip out then the spring comes out. If you remove the handlebars the spring will lift out easily.

"Progressive" brand springs are of reasonable quality and price, most bike shops would be able to order some in.

Thanks Rick G

G'day jkom67 , just reading your original technique to change the fork oil , you said you removed a 10mm bolt at the bottom to drain the oil , I'm no sure that this is the correct process , the drain hole is at the back of the fork leg and is only a 5mm socket head cap screw , happy to be corrected by other wiser heads

John R

Hi John R,

On mine it is in fact a bolt with 10mm head, bottom back of forks!

If I were you I'd look into swapping in some newer K75 or K100 forks from Fleabay or wherever. The early K 100's did not have a fork brace which some of the later K100 models and all K75s had. (There used to be an aftermarket fork brace called Telefix or something like that but those are long out of production.)

I recently redid an 86 K100RS and swapped in some K75 forks with a fork brace.

As for fork oil, I have always had good luck with Honda SS-8 (which is a 10W oil.)

Thanks duck... Will put that into the realm of possibilities! 

Any advice on exhaust mods?? Curious!

jkom67 wrote:Hi John R,

On mine it is in fact a bolt with 10mm head, bottom back of forks!

well ok then , yup the head is 10mm in diameter , just making sure you are not undoing something that you shouldn't

John R

jkom67 wrote:Thanks duck... Will put that into the realm of possibilities! 

Any advice on exhaust mods?? Curious!

Exhaust? Oh my, what a can of worms.

1 - If you bought a 600# bike designed 30+ years ago and your primary concern is performance then you should've just bought a newer 600RR, liter bike or Busa.

2 - If what you want is a loud "look at me" bike then you should've bought a cruiser and removed the baffles. 

3 - If you want to know what's available for an 85 K100 and want my (admittedly biased) opinion then there I can help. I've run several different exhausts on K100s so I have some experience there. Too much to type though.

I only have free long distance in the US but I'd be happy to spend some time BSing with you if you want to call me.  Just drop me a PM asking for my phone number.

Well, new Progressive springs with spacers sized at 2.5" are in with 50/50 5 and 10 weight Spectro oil. The fork still sits at bottom! I pull up and hear a whooshing sound, push down, same sound, she just sits, does not spring back at all. I have rebuilt forks from the 60's to the 2000's never encountered this... 

Any ideas gents?

Missing spacer? Which forks do you have?

When you lift the front up is there any load on the top bung or can you push it down and it stays there.

RicK G wrote:When you lift the front up is there any load on the top bung or can you push it down and it stays there.

There is load, after I installed the new progressive spring and spacers I had to ask someone else to hold the bung down while I installed the clips. It took a lot of effort! When I jack up the front of the K after it has been sitting the lowers/wheel visibly drop several inches.

Some more questions, do the fork legs slide freely when there is no spring in them and just by themselves, no wheel and no fender?  Are the legs parallel, it is possible the tripple clamps are twisted and that can prevent the fork sliding or make it stick in one place mainly when when all is bolted up tight.
Undo the bolts at the bottom of the fork that clamp the axle 2 on each side also release the bolt in the end of the axle and bounce the front end a few times. This allows the forks to find the right place on the axle and makes sure they slide freely (not to be confused with a twisted clamp) then tighten all the bolts and see if there is an improvement.

RicK G wrote:Some more questions, do the fork legs slide freely when there is no spring in them and just by themselves, no wheel and no fender?  Are the legs parallel, it is possible the tripple clamps are twisted and that can prevent the fork sliding or make it stick in one place mainly when when all is bolted up tight.
Undo the bolts at the bottom of the fork that clamp the axle 2 on each side also release the bolt in the end of the axle and bounce the front end a few times. This allows the forks to find the right place on the axle and makes sure they slide freely (not to be confused with a twisted clamp) then tighten all the bolts and see if there is an improvement.

Rick G! Huge improvement with the above described!! 

You my friend are officially a guru in my estimation...!

Oh lord it feels so good to have buttery smooth front suspension!

Much thanks to you!