kz 750 twin excessive vibration

I recently purchased an 82 KZ 750 csi twin and discovered bad vibration just below 4k rpm or 50 mph increasing exponentially so it isn’t resonance of any form; the vibration from the engine gets too bad to ride for longer then 20 minutes. I would like to get confirmation from another owner of the z750 or kz 750 twin that this just isn't a poor engine design (which I can't believe).

If it is something with the engine then some suggestions would be appreciated.

I have cleaned the carbs, spark is fine, engine runs fine and starts up easily. Timing is a little off but can't adjust it on this model, engine mounts are solid, valve clearance isn't excessive. The bike only has 7k miles, so not much wear.

No that is not normal, at least not on the 79, 80, and 82 KZ750 twins I have had and ridden. Maybe try synchronizing the carbs?

My longest non stop ride (without getting off the bike) has been 150 miles, and the only reason I stopped then was to get gas. The mirrors get a little blurry above 50 mph, but I have cheap mirrors

Post edited by: steell, at: 2006/07/17 22:51

Hmmm has it been rebuilt at some time and the balancers are out of synch perhaps. ?

You can't adjust the timing? They must of changed that after 79-80. Hmmmm.
Mine runs smooth, once you fix it you'll love it.

Why can't the timing be adjusted?:blink:

ndali wrote:

...I would like to get confirmation from another owner of the z750 or kz 750 twin that this just isn't a poor engine design (which I can't believe)...

Well, not to slam the bikes I love, but it's not the *greatest* engine design, that's for sure...

Also, I think this is one of those questions of perspective. I'd love to feel one of these bikes that people say vibrates "excessively". I wonder if some people have different tolerances to the vibrations that a twin undeniably has. I mean sure, the way you describe it sounds like ALOT worse than I'd describe mine, but I wonder if it *really* is...

I wonder this, because there's a post like this about once every half-year or so, and everything pretty much seems right with the bike, just the vibration...

And yeah, why can't the timing be adjusted?

Also, have you tried going over 4K RPM? I find that's right about where my bikes kinda smooth out and change personality a bit (getting into the real use of the main jet)...

The timing is just an oval shaped piece of metal that is keyed into place with no adjustment. Then there is a mag pickup which picks up the lobe of the oval. There is a spring which retards the timing at higher rpm. So according to the clymer service manual and from observation, I don't see anywhere to adjust the spark timing.

I am out of ideas regarding the vibration, I don't think the bike has been rebuilt, and it runs smooth till 4k rpm, so I don't think there is anything major wrong with the engine.

I don't think it is a matter of tolerance, this is my third bike and I haven't felt anything like this before. I also work with alot of old timer bikers who have alot of experience and the concensis is that something small isn't right.

If there is another way to adjust the timing then I would like to know so I can adjust it and see if that will help, but I am out of ideas.

you could check the rotor at the end of the crank. It is on a keyway with a taper fit but the bolt has been known to loosen up sometimes. it is part of your charging generator under a engine cover. A gasket may be needed. the slow rpm let it wobble and higher revs the centrufugal force smoothes it out. this is how the stator gets wiped out if it is loose. re tighten with lock tight if

ndali wrote:

...and it runs smooth till 4k rpm, so I don't think there is anything major wrong with the engine.

Ok, I'm just thought of this based on how you just re-worded that...
Besides fixing the timing in general, two things happen as you approach ~4000 rpm
1) your carbs move to the "main" jet system, and
2) your auto-advance should be at full advance.

Regarding #1), maybe you should make sure the main jets are set on the same notch/groove/setting, and that all works properly in the carbs, and that they are clean... you basically could be running on e one carb at that rpm, and that would be horribly vibrating...

Regarding #2), your description of the oblong thing sounds like what I've seen from your model on ebay and such. There's *gotta* be a way to adjust it, I just don't know it. And if the timings off at low idle, it may manifest itself horribly at higher idle. There is an auto-advancer, right? Or is it electronic? If there is one, make sure if moves freely and is lubricated. Can you take a close-up picture and post it?

I'm curious about these '80s twins... I might have to get one....


Post edited by: Biquetoast, at: 2006/07/18 10:08

I just cleaned the carbs and checked to make sure they are fine, the main jets are fine and the jet needles are identical, the idle jet was the only thing that was clogged.

It does have an an auto-advancer, and the timing is off about the same amount at low idle as well as 4k rpm. Off of this side of the crank I am not sure how to adjust it, I took it off and verified that it was lubed and able to move. A thought is to shave down the key then shim it to advance it or retard its position. As I said earlier, a manual for the kz 750 4cylinder states that it is un-adjustable.

I am also curious about trippivot's suggestion, if the rotor was loose that could cause the wobble, but could it be loose in such a fashion as to not contact any other component so there is no other indication?

Sorry to be hopping on this but I have never seen one. The magnetic pickup has no adjustment on it? And you say it IS a little out of time? My brain hurts! I'm not juking you but MAN that would send me over the edge.....again!:blink: :silly: