Make sure that the engine
is set correctly, and then make sure the engine
actually comes back to idle speed when you move the throttle lever to idle.....ie. Make sure the throttle cable is adjusted correctly and pushing the throttle lever all the way to the idle stop.
Then check the adjustment of the shifter and shift cable. When the gear box activation lever is in the neutral position, and the shift lever at the helm
is in the neutral position, the shift cable position must allow the attachment pin to slide into the gearbox
activation lever with out moving the lever one way or the other. If that can't happen then the shift cable end fitting needs to be adjusted to allow the pin easy access.
If all of this is good year might want to get the local VP mechanic
to check out the transmission