83 kz550m1 - shaft transmission question

It's only happened twice in 850 miles but when accelerating thru the gears the up-shift from 3rd to 4th I suddenly find myself revving in neutral.Any idea's what may cause this and /or things to check?

It may be that those 2 times you just didn't kick the shift pedal firmly enough. or maybe you shifted early or late while the clutch was still engaged. ??? if it's not that, then there may be a problem with the gear change drum & forks. The service manual may have a list of possibilities in the troubleshooting section. the 81 650 manual does at the end.

How long have you had the motorcycle & have you changed the oil?

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How long have you had the motorcycle & have you changed the oil?[/quote]

I just bought this bike at the beginning of this year ... oil and filter was changed and filled with shell rotunda 10w-40 diesel spec etc

Finding false neutrals is not uncommon with old bikes like ours. It's hard to say whether or not it's normal or not. There is some variation from bike to bike and model to model. One thing I can say is that if your clutch is dragging too much that can exacerbate the issue. Wearing the right type of boots and adjusting the shift lever so you can deliberately be sure to throw the lever firmly and full stroke can help.

The (chain) 550's all seem to have an easily-found, false-neutral between 5th and 6th, I don't think I've ever found a false neutral between 3rd and 4th on any 550. I can't say whether a shaft drive would be different from a chain. I've mostly dealt with chain-drive bikes.

The shift drum pawl keeper disk may be loose.
Steve

Update :  I've put about 10K miles on this bike since this initial post and just wanted to add that i have pretty much solved the problems with the false neutrals just by modifying my riding Technique.... when shifting i hold my foot up firmly on the shift lever after each shift for extra few seconds and let  off after rolling on the throttle .  now it's like second nature ... no more false neutrals  ... hope this may help someone. 

Weak return spring.
Steve