2 quick quesses, and it may be a combination of both.
1. the throttle plate is carboned up.
2. there is an accel. linkage rod under the air filter behind the fuel distributor that runs across the engine that is spring loaded, and it may be fighting the sticking throttle plate.
Of course I am assuming that your throttle linkage is properly lubricated, all the ball/sockets freely move???!!!

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