These are no longer available from Suzuki so I have had these made by a UK specialist spring manufacturer

I converted my standard RD500 carbs to Mikuni TM32's which are the same carb as the RGV 250 and Aprilia RS250.  The only problem is that the Mikuni TM32 has a very heavy spring and they did not take into account they would be used on 4 cylinder bikes like the RG500 and RD500 so this spring is a sensible upgrade to make the throttle less of a problem for your wrist.

These are a tried and tested upgrade.