I have had a simmiler problem on 00 ZR600EFI. Though I was gitting about 200 - 250 miles per primary spring. This was using the stock A/C 9-post primary. I was using an EPI clutch kit though I belive that the problem is universal. After breaking the two springs My dealer and I started to look a little deeper. The first thing that we found is the spring seats in the clutch were out of spec(to tight) . So the Clutch goes to a local Machine shop to get that corrected. ran 300 miles and snap goes the spring again. Pull the clutch and off to the dealer again. The dealer checked the clutch for ballance, and it was fine. Then He told me to check the Crank Runout, said that it should not be more than .002"; so I setup a dial indicator on and find that it was .0001" out under the spec. Next we tried another clutch to insure that is was not a binding problem Same thing 325 miles later. being a little frustrated I tried a different brand spring that had the same rating (165-300). Snap again. This time Dealer suggested to Try a Sno-Pro clutch. After some recalabration and re-shiming to get it to perform more like the 9 Post unit thiings were better. I went 900 miles without the spring breaking. But it still broke on the last ride of the season.
I have a friend with the exact same sled as I have, he is still running the stock clutching and has had no problems.
After talking with a customer service rep at EPI, He said that the A/C primary clutches do not run long with the heavier springs. Duh... at this point.
The problem is that I like the way this thing runs with my modified EPI setup (added heavier weights). I have an engaugement of 4900 and a max of 8600. Right where it should run. I tried a different setup using a lighter spring and weights and a straight 51 degree helex. This setup went 1200 miles with out a problem, but did not pull as hard.
So the next step is to try a Ti spring from Speedwerx. They say that Titainium is stronger and more durable.
Also, I do like the Sno-Pro clutch better. It is lighter and runs much cooler. It is very simmilar to the Polaris 6 post unit.
Let me know what you come up with.