It would go up in rpm but very little. Let me know what you find. I really should replace mine too.After having my secondary apart several times this season I began to realize I'd lost some life on my secondary spring. Sure enough, I bought a new one and I can tell just squeezing with my hands that the new one is much tighter. Stock spring, 3500 miles on it.
So, what effect on top rpm can I expect as a result of replacing the spring?