The linkage can get sloppy. There are many places that wear can occur, and it becomes cumulative. There's a 3 position spring bracket. A standard universal throttle spring should work if you stretch it from the proper points. You are pushing on a pedal that may be loose if the teeter-totter linkage that transfers the foot pedal movement to go from a push to a pull on the carb linkage. Those holes can be bored to fit a bigger pin, or better yet, bronze bushings.
You should check your choke housing. The presence of soot means you have a hole in your choke tube in the manifold. It gets sucked into tiny passages in the carb as a tar and can hang up the choke linkage and piston in the mechanism.
Barry Wolk
Farmington Hills, MI
C5681126