TOPIC:

I just bought a 1980 KZ1000 (E2?). It has been modified with pod filters and 4 to1 Vance and Hines exhaust etc. I discovered at speeds over 65 mph (around 5000 rpm) it starts to vibrate badly to the point I feel unsafe. The vibration is so bad that I can’t see out of my rearview mirrors, my Speedo/tack is hard to read and my hands go numb. I’m hoping this is not normal, it runs great at speeds below 65.

Check tires/wheels for balance first.

I would also wonder and check the chain and both sprockets for wear.

Tighten engine mounts to spec.

Probably old tires

May not apply to your problem, but some places to check:

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

Motorcycle Wobble 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 runout / damage

Front disc warped - check for runout

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

Not having a front fender, as it helps stiffen up the forks. I’d suggest a fork brace.

Try this - accelerate up to 5000 rpm in 3rd gear then do it again in 4th gear. In each case the bike will reach a speed of something under 65 mph. If the vibration still occurs at around 5000 rpm the problem is most likely engine related. If the problem doesn't occur it probably is related to tires or other running gear rather than the engine since it seems triggered by the speed of the bike rather than the speed of the engine . Ed

Front tire. Too small or just a crappy one.

650ed wrote: Try this - accelerate up to 5000 rpm in 3rd gear then do it again in 4th gear. In each case the bike will reach a speed of something under 65 mph. If the vibration still occurs at around 5000 rpm the problem is most likely engine related. If the problem doesn't occur it probably is related to tires or other running gear rather than the engine since it seems triggered by the speed of the bike rather than the speed of the engine . Ed

thats quite a logical approach, I like it

Had this one mine, turned out to be a badly repaired alternator rotor which was out of balance,

Go with what 650ed said first, then let us know the status. The long list by Jeff Saunders is pro knowledge.

My bike has something of a vibration around 4,500 rpm up to 5k, then it goes away. A local tech said something is rattling at that harmonic frequency. He said it may be exhaust pipe, etc. I may check my engine mounts to see.
Bike should be enjoyable to ride, that sounds really bad.