TOPIC:

Might be engine, might not, but I gotta start somwhere!

Since the coil mod I did this morning was so successful and it being such a nice day (after two days of 50 degree rain!), I took the old twin out for a run up the highway. I noticed before a vibration / roughness about 3500-4000 rpm. I can feel it in the footpegs - almost like really rough road. Today, I tried various combinations of rpm, pulling hard / easy, and it is still there as I pass 3500-4000, but it is also there near 5000 if I pull it hard. Cruising at any range is pretty smooth, it seems confined to when it is under load.

It has me concerned because it really feels like something is going to break! Any clue as to where to look?

mine does the same, the pistons in the twins are up and down together and counter balenced by a large counter weight,unlike a four cylinder where the pistons are up and down at different times,unless its extreme then maybe a timing issue or maybe its ready to explode

or maybe its ready to explode

Ya know, that is what worries me, given where the motor is in relation to my body!

Mine does the same thing, in the 3700-4000 rpm range. I think that there are a few of us here that have mentioned it before. Mine does not seem to have gotten any worse over time. I've had mine running for a while in the 7000-7500 rpm range over the last couple of months, and it still is just the 3700-4000 range that I notice it. Anyone know how fast a 750 twin, with 17 tooth front and stock rear sprockets, is going at 7000 in fourth and fith gear (my speedo cable broke this spring)?

probably fast enough to get yourself in trouble.

agawam wrote:

mine does the same, the pistons in the twins are up and down together and counter balenced by a large counter weight,unlike a four cylinder where the pistons are up and down at different times,unless its extreme then maybe a timing issue or maybe its ready to explode

The pistons on the KZ400 and 750 twins isn't balanced only by a crank counter weight, they are also balanced by two smaller chain driven counterweights driven by the crankshaft. This is known as Kawasakis famous "Harmonic Balancer System". This system doesn't smoothen out all the vibrations though, leaving a rouch patch in a certain rpm area. When the untensioned balancer chain stretch over the time, these vibrations will increase. And if you you can hear a grinding sound together with the vibrations, the balancer chain and the balancer chain guides most likely needs to be replaced.

Some reading:

www.z-power.co.uk/merchantmanager/view_information.php?pId=32 It seems a bit harsh but then you should read what they write about the Z/KZ1000J and B1 (1981-1982GPZ)
www.z-power.co.uk/merchantmanager/view_information.php?pId=35

Some reading:

www.z-power.co.uk/merchantmanager/view_information.php?pId=32 It seems a bit harsh but then you should read what they write about the Z/KZ1000J and B1 (1981-1982GPZ)
www.z-power.co.uk/merchantmanager/view_information.php?pId=35

Geez. Compared to the scathing he gave the gpz, the twin sounds like a dream machine!

Thanks for the histoical education. I have read much about the "twin vibe" here and elsewhere, but since I have no frame of reference (first bike), I was a bit concerned.

Steell - not sure what the 17-tooth sprocket would do, but 5k gets me 70mph. I agree with wireman - trouble indeed!

EsaPro wrote:

....I noticed before a vibration / roughness about 3500-4000 rpm. I can feel it in the footpegs - almost like really rough road. Today, I tried various combinations of rpm, pulling hard / easy, and it is still there as I pass 3500-4000, but it is also there near 5000 if I pull it hard. Cruising at any range is pretty smooth, it seems confined to when it is under load.

It has me concerned because it really feels like something is going to break! Any clue as to where to look?

It could be your coil. However, read on....

I know you're gonna think I'm crazy, but you having this problem may help me also...

You see, both my 750 twins have a strange problem that is exactly as you described, but I have discovered what causes the vibration: the drive chain. When I get on it, just a certain amount, right in that rpm range, for some reason my drive chain will start flapping side-to-side on the bottom. Of course the only way to see this is to look down at it without running off the road...

I can stop the chain flapping just by easing off the throttle a bit (or accelerating harder). I have come to well recognize the feeling in the footpeg, and can feel it coming on, and know how/when to stop it.

Though I know that the chain flapping causes the vibration, I don't know exactly what causes the chain to flap. The only clue I have so far is the possiblity of unbalanced *exhausts*. One bike has stock exhausts with holes that it leaks out, and the other had a slip-on that was leaking at the slip-on. On a hunch, I used some high-temp silicone to try and seal the slip-on leak, and it actually seemed to have made a difference. Unfortunately, my repair hasn't completely sealed it, and the chain flapping hasn't completely gone away, though it is reduced I think.

So, if this is your problem too, that will mean I'm not crazy and I should look into it more aggressively... I don't want to be crazy...

Hmmm, interesting parallels. I have a 2-1 exhaust that was leaking at the slip-on, and I put gasket sealer in/around the joint to seal it off! I think it leaks a bit under the bike as well (There is another joint there, before the 2-1, which is welded). That one does not blacken my pant leg, so I did not do anything to it.

I have not looked at the chain while riding, not sure I want to! I do know the chain is out to the tensioner limit, and I likely need a new one, and probably sprockets also, but I was going to do that over the winter.

It could be your coil.

I do not follow how that could cause this - Misfiring? I was thinking more of a valve / piston type thing, but that is a wild guess on my part.