Intense high RPM vibrations

Hey all, I have a 1980 KZ750-G1 LTD Twin, and this is only my second bike (my first one being a tiny DT80). It always has some vibration to it, definitely engine vibration, but it has really really bad vibration above 4000 RPM to the point that the mirrors are blurred and useless. Highway riding is not comfortable at all. What could cause this? I've played with carb synchronization but nothing I did made an appreciable difference.
Carbs have been rebuilt and cleaned.
Points and Condensor are new.
Ignition timing and point gap were just adjusted.
Compression is fine at 150 PSI per cylinder. What else could cause this?
THanks in advance...

There are guys here that own the big twins and could give you more to trouble shoot with, but... Are your frame to engine bolts to torque spec. (tightened up)? I'm pretty sure that those twins had counter ballancers in them, maybe there is a prob in that area?

Post edited by: luvmykaw, at: 2005/10/15 02:34

I have a KZ750G1 also
My mirrors do get a little blurry above 4k, but not to the point of being unusable, and the vibration is certainly not enough to be uncomfortable.
I assume that sitting on the bike and revving it above 4k in neutral the vibration is the same?
If torqueing the engine mounting bolts does not reduce the vibration considerably, then do a compression test and post the results here. You also might consider checking and adjusting the valve clearance.
Getting into the balance shafts is really a last resort, as you have to remove the motor and split the cases.
How many miles on your bike?

And my standard advice to anyone with a 76-80 750 twin:
Grease the swingarm bushings, the grease fitting is below the airbox, behind the right side frame rail, on top of the swingarm.

Post edited by: steell, at: 2005/10/15 05:05

Thanx for replies - yeah, it vibrates regardless of the gear its in. I torqued the mounting bolts today and it didn't do much to help.

I have not, however, checked valve clearance. Last time I checked compression it was good, 150 psi across both cylinders, but I will test it again to be certain.

The bike has 14.7k on the odometer.

What does greasing the swingarm bushing do? Will it make the bike run like new and give me money?

I'm not sure about the twins, but I find my 4's mirrors vibrate a lot at cetain rpms and get blurry, but you can still tell if someones there. I live and ride in the country, so that's not much of a problem, but in the city I can see you'd want better vision.

My 650 gets a bit too viby for my comfort at 5000 rpm on the highway. So I changed sprockets to lower the rpm. Works great, may be worth looking at if you can't find a mechanical fix for the vibes.

Greasing the swingarm bushing will allow you to keep more money in your pocket
Instead of spending it on repairs, and possibly medical bills

Kawasaki used several rubber bushings on the twins to keep vibration from getting to the rider, the handbars are mounted in rubber bushings, as well as the foot pegs. You might check those to make sure they have not disentegrated. The handlebar bushings are located in the upper triple clamp, and the footpeg bushings in the footpeg mounting bracket. If you can move the footpeg with your hand, then they are bad, and you can look at the top and bottom of the upper triple clamp to see if the handlebar bushings are falling apart.

If I recall correctly, there are two engine mounting bolts on top of the cylinder head, one at the front of the motor, and two at the back, all of those are tight?

150 psi is good compression if the engine is cold, normally, it's 160-175 if the engine is warm.

steell - there's the two mounting bolts that attach the cylinder head to the bracket with the ignition coil, are those what you are referring to? Or did I miss something completely?

The footpegs are probably a little loose, then. I can definitely move them with my hand...

The vibes are definitely the most annoying at the handlebars, though. The rubber bushings look OK, from what I can see.

Rechecked compression today. ~150 PSI on both cylinders after 3 seconds, cold. I still need to check valve clearance, thats one thing I haven't even taken a peep at since i've gotten the thing.

Im getting a vibration at around the same RPM, not that bad that it vibrates the mirrors to much, but it definitly sounds and feels different. Im the same way, Im not going to really be concerned about it until I adjust the valves, and check the chain adjuster. My 82 750LTD has 8400 miles on it, and I bet any money the valves were never adjusted the entire life of the bike. I also get a very small backfire through the carbs at low RPM when accellerating, Im assuming it's a valve or 2 thats a little to "tight".

Yep. the two bolts that hold the bracket that the coil bolts to are also engine mounting bolts. The rear lower mounting bolt is a little hard to see as it is actually under the motor, under and behind the kick starter.

150 psi compression cold is good, it will increase 10-15 psi hot.

The only 750 twin that I have had with a vibration problem is my 750G when I first got it started after buying it on eBay, but it was only running on one cylinder at the time

I'll definitely check that last mounting bolt, and valve lash whenever I get a break from my classes. Thanks for the reponses.

I filled the handlebars will birdshot today, it seemed to help somewhat.