Try having a look at where the throttle cable connects to the injector pump
Have a mate move the throttle control in the cockpit
(make sure the engine is securely off, and the boat is properly moored).
Check that the cable sheath is properly attached to it's mounts both on(?) the engine and at the cockpit
Look at how the cable connects to and moves the lever on the injector pump
It should move smoothly with no binding.
Now check that the return spring on the injector pump has not broken.
If all looks OK disconnect the cable from the fuel pump
throttle lever. Make sure none of the small parts
fall into the bilge
Now get your mate to move the throttle control again and see how smoothly it moves.
Move the throttle control on the fuel pump
and check that it returns to the neutral position when released.
If all looks good reassemble and carefully retest.
This is the limit of my suggestions.
Unless, of course, some bright spark has reversed the gearbox
and throttle controls.