It's a combination of both the severe duty belt,
and the spring...
I have that belt... it grips like nobody's business!
I bought the 300 RPM drive clutch spring, but I never put it in just for the reason that your experiencing,
the belt alone gives me a jolt on take off, I didn't want to add to it!
Tell them you just would like to "try it" with just the belt, and your old spring back in!
