Sounds like we have the same issue. From what I can tell, if you start the machine quickly (while picking up tubers, etc), the motor fires up and the issue doesn't present itself. If you give it gas while cranking it after it sat for 5 minutes? 10 minutes?, then the throttle sticks wide open until you start it, then it races a few seconds once started and returns to idle.
I tell you what, if I figure it out first I will let you know what I did. Feel free to educate me as well. Must be a design issue since they are identical machines. boats.net has good microfiche schematics of the assembly.