Re: What does this spring do?

[david-bartlett]

The spring keeps tension on the cam follower to help bring the throttle plate back to closed when you slow down. There is a spring on the throttle shaft as well, but you should have this cam follower spring. You can get one at a good hardware store.

Oily and black plugs are likely from improper adjustment of the carb, or motor not getting warmed up.