Throttle stuck

[tsubaki]

I'd look at the control cables to see if they might be either binding or need replacing.
Take the control connections loose from the motor, note which one goes where and take care to not revolve the adjusters on each.
See if the cables are then free coming from the control box. If so, grease is needed at the motor portion or depending on the motor there may be a spring that has broken (usually not visible).
If there is still some resistance, you have to go into the control box and determine if it needs greasing, is out of adjustment or if the cables are creating the problem.
Final easy determination if the cable is the problem would be to have it completely removed from the control box and motor to see if it moves back and forth freely.
If one control cable is bad, go ahead and replace both of them. It's just a matter of time for the other one to act up and a lot easier than doing it again.
Post what engine you have in order to possibly narrow it down a little.
Also there is no mention that it may have been adjusted previous. Sometimes they can be adjusted incorrectly and run fine then exhibit the same symptoms you have described.