How do I refurbish a slipping clutch?
This article should be considered a general guide. For anything this involved, you should have a service manual. This is not a frequent problem on the EX250.
The consensus here on the board is that the springs go first. If it's a high mileage bike, the fingers on the basket might have some grooves worn in them.
Remove the clutch cover and see what's wrong. Have a new gasket on hand before doing this, and expect to lose a half-quart of oil if there's oil in the engine. The manual has service lengths for the springs and thicknesses for the plates. If it's the springs, replace them. If it's the basket, file it smooth (take it out of the bike when you do this; you don't want the aluminum filings in the motor).
It's probably not the clutch plates or steel plates; these should last as long as the engine, if not abused. Aftermarket springs are made by Barnett and numerous other companies. These spring kits typically cost around US$10-15, and are considerably cheaper than the OEM Kawasaki springs.
Here's how to get started: Remove the lower fairing to expose the clutch cover and disconnect the clutch cable.
Remove the 9 bolts from around the clutch cover. Make sure to have something to catch the oil that will come out when you break the seal. Turn the arm that held the clutch cable 90 degrees to the left and remove the cover. This will expose the clutch.
Remove the 4 bolts that hold the clutch springs in place. Be careful not to lose the washers.
Remove the springs and outer plate to expose the first friction plate.
The plates will be alternating between steel and friction plates. The friction plates and springs should be checked against the specs in the manual and replaced if necessary. Also check the steel plates, not only for thickness, but for warpage and scoring.
The springs on the left in this photo are original ones that were measured at 0.6mm shorter than the service limit. The ones on the right are new EBC springs that replaced them.
Replace the gasket on the clutch cover and reassemble. Check all torque measurements. Be sure to check the oil now. You will have to replace the oil that was lost when the cover was removed.
IMHO, The OEM clutch springs are soft/weak, and there's only 4 of them unlike bigger bikes like the Zx600, due to this is a beginner's bike according to Kawasaki, to make it easier for beginners male and female. This leads to slippage in certain cases, especially at high RPMs
This is an easy fix, replace with aftermarket performance springs.
Choosing Clutch Springs
Barnett aftermarket clutch springs for a 1985-1989 ZX-600, Part # 501-50-05055. You can view them online at the Barnett catalog, here: http://www.barnettclutches.com/products/clutch_plates_springs.aspx?type=clutches_other&clutch_make=Kawasaki&clutch_cc=600 They can be ordered through your local dealership, or at a number of online sources. Here is one such online catalog: http://www.exoticsportbike.com/6001.htm
Another option is the EBC CSK10 (see http://www.ebcbrakes.com/motorcycle_clutch/sck/clutch_springs.shtml). This kit isn't quite as stiff as the Barnett, but is better than stock and usually a little cheaper. The kit does include six springs, while the ninja 250 only uses four.
I have done this to both my Ninja 250 & EX 500
The clutch pull is a little bit more stiff, but nothing to worry about. The shifts have more of a crisp and positive feel, and eliminating slippage is always a good thing when it comes to the clutch.
It's my personal opinion that this is a must do item, as it saves the clutch itself, and eliminates a possible problem especially when the springs get older.