forks

[franck]

I have recently renovated my K75C (1985) and been riding it for a few months. The K75C has a some time been fitted with a full fairing possible of a K75RS.  The bike runs well, and no problem in cornering - however on rough road surfaces I get quite a bit of front end vibration - most of the roads in my area a surfaced country roads and in places heavily patched.  Should I use a more viscous fork oil (at the moment 10w)  or could this be the start of a fork rebuild?

[rbm]

If by "shake" you mean the forks don't follow sharp bumps in the road very well, then the high speed compression damping is too high.  Use a less viscous fork oil.  Something like 20 Cst (usually a 5W oil).  There is very little you can do with K75 forks - just set preload and play with oil viscosity.  There are no rebound damping control or high speed valving like modern sports bikes.

[Scott_]

What do you have mounted for a tire(type/brand) and at what pressure.......

[franck]

Tyres mounted on the bike
Make Bridgestone     front 100 90 18   Rear 120 90 17    (drum brakes)
I keep the tyre pressures a bit above recommended as I have had some pressure loss due to leakage at the level of the rims (the bike was left standing for some years)   Pressures    front 32 psi rear  36 psi

[Martin]

Did you put new tyres on the bike when you put it back on the road.
C.heers Martin.

[Laitch]

I keep the tyre pressures a bit above recommended as I have had some pressure loss due to leakage at the level of the rims (the bike was left standing for some years)

If those were the tires on the bike when it was standing for years, they may have deformed. Replace them before you do anything else.

[franck]

They are new tyres - it was the first thing I did after clearing out a nest of mice from the air filter chamber and general heavy maintenance.   I think the problem with the tyres losing pressure was corrosion on the rim.  I had the tyres remounted and this time I took some pains to make sure the contact surface was cleaned up. 

[Martin]

Try  raising 2 PSI front and rear.
C.heers Martin.