Lots of good suggestions here. Having been through a sticky throttle journey of my own and having (finally, hopefully) resolved it.
The hand throttle can create issues. Phak1 makes a good suggestion. Make sure the hole that cable runs through in the vertical rod doesn’t have grooves. Also, too long of a “tag end” on that cable can grab the top of the manifold at times.
Jerry’s idea about the motor mounts is definitely worth exploring.
But if both of those things check out….
* it won’t be any fun, but I’d dissamble everything from the pedal to the carb and have a look.
* with all the moving parts, even just a little bit of grooving and/or egg shaped holes….all that friction adds up.
* it would be an easy enough thing to move the spring connector one position rearward. It’s just underneath an oil pan bolt. I don’t think that’s your problem, but it probably shouldn’t be rubbing.
I can take some pics of the “rebuild” I did to my linkage later on. It’s way, way smoother now.
