From MopedWiki

This article is a wikified version of this post from the forums.

This spring was $4.99 at my local hardware store. Made of stainless steel. Great quality. The package says # C-806 from the Centruy spring company, and the measurements are 38.10 × 82.66 × 3.769mm.

Comparison to stock derbi 3 weight spring:

Installed in derbi rear pulley cam:

Results: In a Metra 80 piston port, it has great low end -- pretty much spot-on with what you would tune a TJT or 6 weight to. I also machine my 3 weight variator cheeks down to allow it to variate further. Didn't loose any top speed from this.