Lewis...
...I've covered this before because it happened to me twice. The throttle lever, where the cable connects on the engine, can stiffen up if it get wet. Check this by disconnecting the cable then working the throttle lever on the engine by hand. It should move very freely and spring back to the original position because it is spring-loaded. If it is stiff, spray it with penetrating oil (I prefer a product called PB) that can be acquired at an automotive store. While you are there, also purchase a can of spray lithium grease. Let the PB set overnight then liberally spray the whole mechanism with the lithium grease. This is the same stuff that the auto dealer uses when they lube the hinges on your car door. It will set up a moisture barrier and should be good from here on out.This happened to me twice. The first time I only used WD-40. That's why it happened a second time.While you have the cable disconnected, try the cable also to determine if it's the real problem.