Topic: 750 K7 fork stiff

[Erny]

Today did another 160miles and definitely confirmed fork is too stiff, it does not "eat" bumps, moves just little or slow, as result i felt all bumps in bars...
Of course when standing, if I press brake and I push bars it moves, this looks normal.

Any idea what except f checking right amount of fork oil i can check? PO changed seals and put new oil.

[Bankerdanny]

What weight oil did you use and how much do you weigh?

At 240-250lbs (108-113kg) I use 15wt oil.

[Erny]

PO told me he pit there Motul 10w. My weight is 80kg

[Yamahawk]

PO told me he pit there Motul 10w. My weight is 80kg

Erny,
  Check the level of the oil in your forks, to see if they are too full. Sometimes it will stiffen up the forks a lot if the oil level is over the recommended height in your fork tubes. Mosul 10w should be fine for your weight.
Charlie

[Erny]

PO told me he pit there Motul 10w. My weight is 80kg

Erny,
  Check the level of the oil in your forks, to see if they are too full. Sometimes it will stiffen up the forks a lot if the oil level is over the recommended height in your fork tubes. Mosul 10w should be fine for your weight.
Charlie

That's exactly what I plan to do as the only thing I can

[BobbyR]

This is a long shot, people sometimes put a spacer under the top nut to compress the springs to make up for fatigue or stiffen the forks. This of course limits the travel and make them bottom out easier.

One other thing. is this a perception on you part about feeling the bumps? I am not sure of your riding experience or the bikes you have ridden. 80kg is about 160 lbs, I admire you. 

[pjlogue]

When I restored my 750 I put in 10W-30 and found my forks were too stiff for a comfortable ride.  I'm 210lb.  I switched to ATF and it made a big difference in ride quality.  My riding style is laid back and not pushing the bike.  If your oil levels in the forks are correct and you have normal (not heavy weight springs) and no spacers give the ATF a try.

You should also check for wear of the forks, binding/stiction of the seals.  Some seals can be pretty tight against the down tubes and cause a harsh ride.

-P.

[Erny]

When I restored my 750 I put in 10W-30 and found my forks were too stiff for a comfortable ride.  I'm 210lb.  I switched to ATF and it made a big difference in ride quality.  My riding style is laid back and not pushing the bike.  If your oil levels in the forks are correct and you have normal (not heavy weight springs) and no spacers give the ATF a try.

You should also check for wear of the forks, binding/stiction of the seals.  Some seals can be pretty tight against the down tubes and cause a harsh ride.

-P.

What is ATF? (sorry for stupid question)

Yes seals is another story, PO have put new might be issue too... Maybe I shoud preventively replace them by OEM ones of they are still obtainable.

[Yamahawk]

This is a long shot, people sometimes put a spacer under the top nut to compress the springs to make up for fatigue or stiffen the forks. This of course limits the travel and make them bottom out easier.

One other thing. is this a perception on you part about feeling the bumps? I am not sure of your riding experience or the bikes you have ridden. 80kg is about 160 lbs, I admire you. 

Yep, this does happen. There is also people who cut the springs to stiffen them and then fabricate a spacer to add back then length they cut. It's a common mod on the Concours forum, but it limits travel somewhat as the coils can bind if a heavy pothole/bump is encountered. The Best Way to take care of forks, is to properly rebuild them with new, straight wound springs, from RaceTech, Sonic, or one of the other popular brands, and use a proper weight fork oil for your weight. Progressive doesn't make a bad spring, just use RaceTech's spring rate calculator to get the proper load rate for the new springs. If they sell you the factory spring rate, they were already too light when new, and then go downhill from there.
Here is Sonic's spring rate calculator...
http://sonicsprings.com/catalog/calculate_spring_rate.php

and here is RaceTech's... make sure you pick street bike
http://racetech.com/VehicleSearch
   ...And ATF is automatic transmission fluid
Charlie

[Erny]

Thanks,  Charlie

I expect that springs are stock original, bike has only less 9000miles from new. But I did not opened then yet. Will do and let you know.

Concerning experience, I have experience with Honda CBF1000F (2010,model SC64, nothing special, just normal tourist bike with low cost forks) having very similar weight.
Simply riding CB750K I have feeling that forks does not move unless there is bigger bump or when braking with front brake. When looking at forks during ride I do not really see them moving, but I feel all bumps in my bars.
On the same road that I know for 20 years previous bike fork was working normally,it was simply moving significantly during ride.

And to add I also have BMW R1200GS LC 2016 that is of course completely different riding and feeling

[Yamahawk]

Erny, the forks could be suffering from Stiction... try this. Loosen the front fork caps, and then pump the forks with the brake on. That will center the wheel in the forks, if it is out. Then tighten the fork caps.  Then, your triple trees can be done, loosen the upper right pinch bolt and lower left, pump the forks again, (re-tighten the bolts) then do the lower right and upper left to take out any misalignment in the trees. Remember to tighten the first two bolts before loosening then other two, or your forks will slip up into your trees... There may be a method to do this in the SOHC manual, also.
Charlie

[Bankerdanny]

When I restored my 750 I put in 10W-30 and found my forks were too stiff for a comfortable ride.  I'm 210lb.  I switched to ATF and it made a big difference in ride quality.  My riding style is laid back and not pushing the bike.  If your oil levels in the forks are correct and you have normal (not heavy weight springs) and no spacers give the ATF a try.

You should also check for wear of the forks, binding/stiction of the seals.  Some seals can be pretty tight against the down tubes and cause a harsh ride.

-P.

What is ATF? (sorry for stupid question)

Yes seals is another story, PO have put new might be issue too... Maybe I shoud preventively replace them by OEM ones of they are still obtainable.

ATF = Automatic Transmission Fluid. The owner's manual actually calls for ATF rather than fork oil on many of these bikes. I think it was because there wasn't fork oil available at the time and ATF resists foaming. If I recall correctly standard ATF is about the same viscosity as 10wt oil.

[RJ CB450]

I havr progressive springs in one bike, nice spring.   That said, talk to recetech if you like to put the miles on.  I am doing a full racetech on one bike, and customer service alone has me as a loyal customer to them now.  Get them your weight and riding preference, they will hook you up.  Then there are the gold valves through them too if you got the forks off :-)

[Yamahawk]

Using the Sonic spring rate calculator for 165lb. and 550lb. bike, for standard/sport touring  bike and normal riding, with a tank bag option, spring rate is .95Kg.
This is compared to the stock spring rate of .65 or even lighter... given the 40 years sitting on those springs, even with only 9k miles on them they will have spring sag, and you can see they are horribly under sprung, like every street bike out there. I am sure you stiffness isn't due to too stiff springs, but of fork oil viscosity and height, and fork seal stiction. Once you get those forks sorted, with proper damping and spring rate, you will find it rides like a modern bike... and if you stick emulators in there, it will be riding very well indeed.
Charlie

[PeWe]

My fork CB750K6 work extra good this season since I replaced the plastic piston rings inside. I have never replaced them before.
Item no 9
https://www.cmsnl.com/honda-cb750k7-four-usa_model14395/partslist/F07.html#results
Those pistons did just fall free inside the fork tubes before. Now they have do be pressed down.

I replaced the rebound springs too when at it. Item no 5 .I did not feel much difference when squeezing the springs by hand, old vs new.
I have progressive springs with no spacers.  Fork oil SAE 10 160mm from top fully collapsed without springs.
It work much better for the small bumps, before too soft.

[Stev-o]

PO told me he pit...

Dont ever believe PO....drain the oil

[tlbranth]

Read this blog on Randall Washington's GL1000 site. It's a technique for removing 'sticktion' from your forks. I use this series of steps on the GL1000 and the CB750. It may solve your problem.
http://www.randakksblog.com/removing-stiction-from-gl1000-front-forks/

[Erny]

Read this blog on Randall Washington's GL1000 site. It's...

Thanks!
Very interesting procedure, indeed.

To apply it on 750K7,do you know by chance what shall be level of air over oil in fully compressed fork? I assume GL forks are different from CB750K ones.

[tlbranth]

On the CB I just put in the recommended volume of ATF. I don't measure depth so don't know the answer. Sorry.

[Desert-SOHC]

When I rebuilt the forks on my K8 I used ATF and they were to stiff, I ended up with 10wt fork oil to get it to ride decently.