KZ400 D3 vibrations come and go

Hi!

I recently finished restoring a ‘76 Kawasaki KZ400.
I did a extensive rebuild, except for the engine: I changed wheel bearings, cleaned the carbs, electrics, new sprokets, new plugs, new coil…

Kept it mostly stock, except for the air filter which has been swaped for a pair of K&N filter pods.
 
Carbs have been throughly cleaned and properly sync, cam tension is correct, so is the ignition timing, valve lash is within spec, compression is perhaps a bit on the lower side (last time I checked was 125 and 115 PSI cold) but within spec nevertheless. Carb vacuum is perhaps a little low (20 cm Hg) (manual states 22-27 but then again I’m using some cheap gauges). Charging system is good as well.

The bike has stable idle and has plenty of power. Had a look at the spark plugs after some miles and the bike appears running just correct, not reach or lean (typical pod problems).

I also checked the engine mounts, fork, swing arm and every single bolt on the bike and everything is properly torqued.

The problem I’m having is that I’m getting HUGE vibrations that come and go with no apparent reason. These vibrations don’t seem to be rpm, gear, temperature or speed dependant. They just appear and disappear randomly.

I may be cruising nicely, holding the engine at stable rpm and out of nowhere the bike may start to shake like hell. To the point that my hands and ass go numb and the bike is barely rideable.
Then, after some miles, all of a sudden, the vibrations may disappear and the bike start to run so smoothly that you would’t tell whether the engine is even running or not. As if you were riding a bicycle.

The bike normally runs a little rough compared to the smoothest state, but not as bad as the shaky state. It keeps giving power nevertheless.

This also happens at idle, sometimes it’s shaky, sometimes remains super smooth.
There are times that the bike cold cranks smooth, and others not.

I have tested whether the bike is running on one cylinder when this rattle happens, unplugging a spark plug cap
at a time, but both of the cylinders seem to be working.

I’m a little bit confused because if it was an engine balance problem or compression related, the vibration shouldn’t come and go, right?
The same if it was an ignition timing problem, or any electric problem.

My only guess is that it can have something to do with the carburetors, which are not delivering a contant mix of fuel and air but I have inspected and cleaned them twice.
In addition, what could be causing this random behaviour?

Any ideas?? 



 

I took a video showing how the vibrations go all the way to the front wheel.

youtube.com/shorts/Fm0Q3lNH-L4?si=rjWhOTp5anQBuEui

Is there any damage to any of the "rotating" components such as generator rotor, clutch basket etc, also as I understand it the primary drive is chain ? what condition is that in ?

Wookie58 wrote:

Is there any damage to any of the "rotating" components such as generator rotor, clutch basket etc, also as I understand it the primary drive is chain ? what condition is that in ?
 

Hi, thank you very much for your reply.

The generator rotor and the alternator side seemed in good shape last time I looked at.

I haven’t had the chance to have a look at the clutch and primary drive (chain indeed) side though…
At that time I wasn’t sure about messing with it and having to buy another seal unless it was completely necessary.

Do you think there might be something loose in there?

It’s possible that the problem is wear in the primary chain or clutch basket, would be good if one of the 400/440 guys voiced an opinion before you dive in there. I don’t know what is “normal” for one of these

Your engine sounds right. Don't mean to come across as trying to teach you how to suck eggs mate but have you checked your head steady bolts? They can come loose. 
With old British twins, when the crankshaft goes out of spec they sort of walk themselves forward when on the centre stand. If you have a centre stand or if you have a motocross-type lift stand, you can see if that's what's happening while the vibration occurs (see if the centre stand is trying to relocate). Maybe try that. In your video, the front tyre judders forwards and backwards. Your chain is also moving due to the viscosity of the oil. If you pull in the clutch, does the vibration go away?

Michi wrote:

Your engine sounds right. Don't mean to come across as trying to teach you how to suck eggs mate but have you checked your head steady bolts? They can come loose. 
With old British twins, when the crankshaft goes out of spec they sort of walk themselves forward when on the centre stand. If you have a centre stand or if you have a motocross-type lift stand, you can see if that's what's happening while the vibration occurs (see if the centre stand is trying to relocate). Maybe try that. In your video, the front tyre judders forwards and backwards. Your chain is also moving due to the viscosity of the oil. If you pull in the clutch, does the vibration go away?

Hi!
No worries man! You are talking to a completely newbie here

The first time I run the bike after I completed the rebuild I realized it was leaking between the cylinder head and the valve cover. I put a new seal (and now that you say it I painfully didn’t take my time to check the cylinder head bolts torque, I only torqued the cover bolts that I removed) only to find out later that it was leaking through the head gasket as well. But I’ve read that’s a typical KZ400 problem anyways. Maybe you are right and cylinder head was not properly torqued from the start  

The bike does walk when on the center stand xD but she wants to go backwards.
About the chain, it keeps shaking when I hold the clutch, and so does the hole bike haha
To make myself a little bit more clear, the bike shakes in the same frecuency as the engine, but it happens (and not happens) at any rpm, not just in an specific range (2000-4000 for example).

I think I will try to do a wet compression test to narrow down the source of the problem. 

Thanks for the hint!
I’ll keep you updated

i had a 74 model and it would go the same thing shake like hell then shut it off and recrank it and it ran smooth as glass.it sounds to me like the counter weights in the engine to me,,,hopefully that helps