It could be stepper, they can stick in the housing try inspecting it before you buy one. They live underneath the airbox but enters the airbox At the right hand side. You can feel if it is working by pushing gently on the piston and turning ignition on. On the walbro it fully closes when the ignition is turned on and backs down slightly to running position. Other things are leaks on the stepper hoses or throttle bodies .faulty TPS. Have you changed anything lately TPS reset, manual throttle adjustment , injection correction?