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I could be completely wrong, too. I'm planning on buying that set of springs.

No eBay APP ID and/or Cert ID defined in Kunena configurationYes, the Z1E springs are K&L which are the same as stock springs. The EBC are a bit stronger, about 10-15% as I mentioned. They are also about the same price. Here is a set.

There are many other places you can get the EBC springs and if you talk to Z1E and they do not have them, they can get them for you at no extra cost. They are great people to work with.
EDIT

Z1E has the EBC springs CSK7
www.z1enterprises.com/detail.aspx?ID=2920

Changing these springs every few years is a good idea. I would say about every 3-5 years. Heck, you spend more money on an oil change. I also change my fork oil once a year also. A bit off this subject of clutches, I use Synthetic ATF in my forks with a bit of Lucas Transmission Additive to make it a bit more viscous. I like my fork fluid about SAE-30 equivalent.

Wow, they are cheaper than the K&L's. Very cool. Thanks again, Otakar!

I updated / edited the previous post.

Since I wasn't able to find them on Z1E I went ahead and ordered some from
bikebandit and they were about the same price. Now I just need to check my steels so I know whether or not I need to order those as well. They look ok but since I actually had stuff break I want to make sure just in case.

Z1E has the EBC springs CSK7
www.z1enterprises.com/detail.aspx?ID=2920

There is some good info in this thread, and I know others also have this problem, so it should be ok to revive this awesome info

. I also have experienced some clutch slipping when cold, but it seems to subside when the motor gets hot. When I did the rings during the infamous leaking head gasket and O-ring fix a few months ago, I had during a brain fart, used car oil for break-in. I have since switched back to Rotella T-6 after ~500 miles, but the plates are well lubed now with molybdenum. I assume the clutch is coated with the moly additive from the car oil. and my springs are also 36 years old.
The springs spend their entire life in the compressed state, so are bound to lose X amount of spring pressure after numerous heat cycles.
I found this video online of coil spring manufacturing. These are automotive coil springs. but the manufacturing process for valve and clutch springs has to mucho similar..

 I don't know what I'm doing wrong with the Kzrider forum search engine, It said -0- results in the search engine.. this thread did not pop until I did a google search with Kzrider at the end of the search terms "refurbishing your clutch". THEN there were hundreds of hits. go figger. anyway, in light of all the latest slipping cuitches in recent topics.. Bump. lol

Barnett makes a good clutch spring too and I've gotten a few sets off of Ebay........ a fair amount of folks smarter than I recommend the nos springs and say the heavier springs are harder on the hands but I've never noted enough of a diff to matter and like the extra holding power. Stock KZ plates are defently the way to go though.... almost all the fast and big powered guys on here use and recommend em.

KZB2 650 wrote: Barnett makes a good clutch spring too and I've gotten a few sets off of Ebay........ a fair amount of folks smarter than I recommend the nos springs and say the heavier springs are harder on the hands but I've never noted enough of a diff to matter and like the extra holding power. Stock KZ plates are defently the way to go though.... almost all the fast and big powered guys on here use and recommend em.

 The only way to really tell if a clutch spring (or any spring) is to compress in on a spring scale and actually measure spring tension at the exact height prescribed. A spring scale can be rigged up using a cheap scale and a drill press, arbor press or vice and a ruler or a spacer cut to the exact length. 

 I bought the K&L cheapies, and in light of not having the proper spring tension measuring equipment myself, it's a guessing game as to how good springs are, or aren't. I assume the fast, big power members have the proper equipment to make their bikes run as fast as they do.

 The spring scale in this video is pretty pricey, but there are other options. This is just a little FYI for folks that don't know how springs are "weighed".

 

Kawasaki Z1 has a standard for spring length in their manual.