Engine vibration between 4000 and 5000 rpm

Hello,

I have a 1984 ZN750 Ltd which is giving me some vibration when approaching 4000rpm but disappears at 5000rpm. Anybody have any ideas why this would be? The carbs have been competely cleaned and tuned (Although I don't have an experience with the guy who tuned them before so I can't vouche for his skills). I will be throwing in new plugs after this weekend.

Any help would be great! Thanks.

Curtis

You might want to check your engine mounts. I had a similar problem on another bike that I had. It had a vibration only at a certain RPM range and it turned out to be the bolts for the rear engine mounts had loosened and 1 lost the nut completely. This caused a vibration that was only noticeable when on the throttle and only for about a 2K RPM range once I had everything torqued to spec the vibration went away. Just a thought. I hope you can figure out what the issue is

I checked all the engine mounts and the're all tight...although I wish that was the problem. I think I have narrowed it down to one of two things:

1. Timing chain and tensioner? I heard these can be an issue for these motors. Any thoughts?

2. Exhaust header bolts. I put a new set of headers on the bike recently and although I torqued them down to the heads, when I took them off today (for other reasons) the bolts were defintiely not tight. I think that when I originally torqued them up I only ran the bike at idle for a minute or two before re-tighting them. Then when I actually went out and ran the thing on the roads they should have be tightened again...could this cause the vibration? headers slapping against the exhaust ports?

The vibration is a fairly high frequency and is definitely coming from the engine.

I have replaced my cam chain tensioner with an APE manual one and I just installed the Liska racing billet idler gear because the last time I had the valve cover off I noticed a lot of play in the stock one. I was getting some noise when I would first get on the throttle in top gear so hopefully this will take care of that. I haven't had a chance to ride it since the very last valve cover bolt I torqued stripped and I haven't had a chance to fix it yet.

I'm going to check the exhaust header theory first...then if it persists I will pull the valve cover and take a look at the timing chain and pullies. Is it quite obvious if something is incorrect with the timing system when you poke around under the valve cover?

I wish you the best with fixing the stripped hole and thanks for your replies.

Curtis

My 78 kz650 does it, my 2005 Ninja 500 does it. Just seems to be the RPM range that closest matches the Kawasaki's natural frequency.

On my 650 it is the tank and bars, on the ninja it is the tank and fairings. As a rule I try to stay above or below that range just because of it.

Big_Wheel, is this bike new to you or have you owned it for a while? Is the vibration a new thing? Most I-4's have a rpm where the vibration resonates. My zx-10r vibrates at abot 8000 rpm, but you never need that rpm at legal speeds so I either stay below it or accelerate through it as fast as possible.

Thanks for the replies. This bike is new to me so I can't speculate on whether its a new vibration or not. I just don't want it to be something serious. It's just a little annoying since the vibration at 4000rpm lands right in the middle at 80km/h (rural road speed limits) and disappears at 60km/h or 100km/hr. I wish I could change the rear end ratio but its a shaftie. I could also fab up some foot peg isolation brackets and fill the bars with chaulking. Add some sheep skin to the seat and it should be okay for me...but I'm more concerned with the engine's health than my comfort.

I did some reading about rubber isolation blocks that go between the cooling fins...any experience with them guys?

I still haven't had the chance to re-install the headers and see if it helps having them torgued properly...final exams are here...

As mentioned above, all engines have a natural frequency when they vibrate a little more. Used to have a Yamaha XJ650 Maxim that had a rash at about 4300 rpm. Even magazine testers reported on that one. My Zephyr 750 by comparison, does not have that rash, but is by no means as smooth as a car.

The rubber blocks are to dampen the natural harmonics of the fins. My zephyr has them but don't think they would make much difference. Check all things and bolts are tight.

Heavy bar end weights can help, also a better quality (heavier) bar. Torque the engine mounts down tight. On my 650 it hit right at about 50 in top gear so I had to move through it rather than hang out and listen to the buzz.

My KZ650 does it, my Z1 900 does it, even my Suzuki GS1100 does it. It sure sucks when your fingers tingle from it.

Thats guys for the replies.

All the mounts are tight. I am taking the bike into a local shop just to see if its a normal vibration. With my limited experience I can't really tell if its something more, and like I said, all I really care about is that its not harming the engine. If it turns out to be the natural harmonic frequency of the engine I'll live with it...if I really cared about comfort I would have bought a newer bike. I really appreciate the time you guys have spent responding to my concern.

Curtis