TOPIC:

Factory sprockets for your bike are 13/32, giving a 2.46 ratio. Your current sprockets of 15/40 are a 2.66 ratio. Lower gearing (higher ratio number) will give rapid acceleration at the cost of top speed. Think about riding a 10 speed bicycle in first gear. You can take off like a rocket, but you'll soon be pedaling your tail off and not going very fast. Changing to a larger counter sprocket would be the cheaper option, but there may not be clearance for a 16T (16/40=2.5). A 37T rear sprocket (37/15 =2.46)would match the original factory ratio for your bike.

A 15/40 is significantly different than the stock setup. plus, you're running a smaller tire, which makes it higher revving, perhaps adding another 150 rpms at 60 mph.

By IOC750: I must add that there is no problem with acceleration and/or pullaway. It does not feel like overall power is an issue

The bike cannot match freeway traffic, that would be a one way ticket.

I'm with Ed that the clutch is slipping. The clutch release and cable are good.

I'm thinking that the five probably-38-years-old springs on the clutch spring plate are no longer doing a good job of holding/releasing the clutch assembly. Is there merit to this train of thought?

Your clutch springs may be fine. My KZ650-C1 is 40 years old with 60K miles on it, and the clutch springs are still fine.

However, I did replace the clutch plates 15 years ago. This was necessary because Castrol had changed their GTX 10-40 oil formula and started using some "friction modifiers" that coated the surfaces of the friction plates which made the clutch slip. That's when I learned about using oil that is JASO-MA compliant. In my case I believe the oil problem took a couple years to hurt the clutch disks. The thickness of the disks still were within spec, but the friction material had been ruined. Ed

I suspect the clutch has been slipping and the PO lowered the gearing to compensate for it. I'd go back to stock sprockets and if the clutch slips, replace the clutch.
Steve

Also, it's quite possible that the previous owner used energy conserving car oil in the engine and ruined the clutch plates. Replacing the wrong oil with the correct oil will not cure the problem once the clutch plates are damaged. Ed

I wonder if they could be scuffed up using 320 paper?
Steve

SWest wrote: I suspect the clutch has been slipping and the PO lowered the gearing to compensate for it. I'd go back to stock sprockets and if the clutch slips, replace the clutch.
Steve

As has been pointed out in this thread, the current gearing is at a ratio of 2.66 as opposed to the stock 2.46 - I should still be able to reach about 180km/h with the current setup but I'm struggling with 100km/h - taken from relative traffic and experience.

It is very likely that a PO had used regular motor oil at some stage and caused plate damage.

I will drain the oil and check the plates over the weekend.

Try to put it in 5th with the front tire against a wall or something and see if you can make the clutch slip or make it die.
Steve

SWest wrote: Try to put it in 5th with the front tire against a wall or something and see if you can make the clutch slip or make it die.
Steve

I'll give that a shot

One thing I've just remembered - sometimes when the motor is still cold, right after starting, the engine will die when shifting into 1st gear. This is circumvented by giving just a little bit of throtle while gearing. Sound familiar? Once I start to ride it is fine though.

Be careful not to apply too much throttle. You do not want to bend the forks. Ed

Z1 can tell you the diff between a 630 and 530 chain set up......I'm thinking your set up is close to stock 630 vs 530.....but check to be sure..... I had a 16/41 530 set up with the 26.5 inch (approx) rear wheel it does about 4.500 at 60mph.... with a 15t front it goes to approx 4,950 maybe 5,000...... you rear tire is approx 24.5 to 25 inches (brands vary to much to know for sure) so that will lower it more but like you say I don't think as much as you are getting. If you do decide on a new clutch I'd go with oem plates and I'd replace the springs.... after heating and cooling for 38 years they are cheap to replace. Try to see what Z1 says first before spending ?

I like Steves idea to check the clutch too .... maybe a big hill in 5th and apply a little brake under full throttle ?