High speed wobble.

I must be bad with the search feature or something because I can't find much on this.
My 1976 KZ900 rides extremely well. I throw it around in the mountains and I can keep up with modern bikes quite well. Absolutely rock solid at the limit through corners. The bike is perfectly smooth to around 115mph. At around 120 it starts to wiggle a bit and I am too scared to push past 125 due to it getting worse.

Is there a cause and fix for this?

What have you changed on it?

This is a known problem of a old Kawasaki. They did this when new. I use to brace my elbows against my knees to work through this. You are lucky the bike does not have a head shake when exiting corners on the gas and hit a section of pavement that is not level.

High speed stablizer and good maintaince is all I could advise. Roy

Your lucky most KZ900/KZ1000's start to wobble at just over 100 mph.
Mark

wireman wrote: What have you changed on it?

Nothing other than slide the forks up the clamps about half an inch. The bike is bone stock except for the Kerker and I run a capacitor instead of a 11 pound battery.
I have brought the bike out of corners sideways and never noticed anything unusual at all. The bike does lean to the left when I let go of the bars but I figured that I just had the rear tracking set slightly wrong.

I guess I will start collecting pieces for a stabilizer and keep it under 120 until then

Dropping front of bike without dropping rear same amount lessons trail in front end,which speeds up steering but gives up high speed stability.
This is the one situation where Id actually agree with a front stabilizer if youre going to leave it that way,probably wouldn't hurt to tighten up steering head bearings also

I think there is a stabilizer shock absorber available for some makes and models of streetbikes. maybe there is one available for custon fit.

Here's where I would check:

1. Tires - they can look perfect and still cause a high speed wobble, also make sure tires are balanced and pressure is set according to Kawasaki spec. If tires are worn replace them. If no other action in this list eliminates wobble replace the tires even if they look ok (guess how I know!) Make sure wheels are not damaged at all.

2. Shocks - if yours are original throw them in the trash - they are shot; if they have been replaced make sure they are high quality units in good shape or else replace them.

3. Swingarm bushings - if you don't know when they were replaced you should replace them (even a little play in them goes a long way to mess up handling)

4. Steering stem bearings - if yours are original replace them with tapered roller bearings; make sure steering stem torque is in spec

5. Fork oil - if not fresh replace with Bel-Ray 15w FORK oil

6. Make sure drive chain is adjusted correctly and chain/sprockets are not worn out

7. Make sure there brake rotor(s) are not warped and dragging unevenly

Also, I would try resetting the forks to the stock height to see if that eliminates the wobble.

CrazyDuck wrote: The bike does lean to the left

strange that mine has trouble leaning to the right.

I copied this from a post from Jeff Saunders (owner of Z1 enterprises)

Motorcycle Wobble/Vibration Causes

There's a LONG list of potential area that can cause this.

These are not in any particular order

Wheel bearings

Steering bearings - repack grease, check tightness

Swing arm bushings/sleeves/bearings - replace if worn, repack grease

Fork springs aging - check spring length

Unequal fork oil - if seals leak, then your damping can be ineffective

Rear shocks aging, damper leaks - if you have oil leaking from the seals, the damping is toast.

Worn Tires/Mismatched tires/tire pressures/wrong size tires

Wheel alignment - don't trust the chain adjusters - I visually align the wheels.

Bent rims/broken spokes - get the wheels off the ground and spin them - check for run out / damage

Balance the wheels- makes a big difference.

Check for wheel run out(out of round).

Tire not fully beaded(seated on the rim).

Low air pressure in either tire.

Front disc warped - check for run out(out of round)

Dragging brake pads

Steering & swing arm bearings are very common problems, but so are tire issues (pressure/mismatch) and wheel alignment.

for some reason my swingarm didn't seem straight, I measured shock to rim on either side and I was wrong.

Thank you very much. Looks like I am moving a few upgrades to the do ASAP list.
I guess I need to find some good rear shocks. Mine don't have that many miles on them but I didn't really like them anyway.

Rear swing arm and brake upgrade is coming very soon. Have everything but a master cylinder and sprocket. Stupid bushings are starting to show signs of wear with not that many miles. Swing arm might be bent too.