40 Horse Power Motor. Starts in starting position, when bringing it down to neutral it stops. It won't start?

it stops. It will start up again when you unplug the gas, then plug in again when it starts. A lot of smoke is coming out all the time. It runs at med speed even when it is in high start. Added gum out thinking it is varnish in the carburetor. We let it run for about 15 mins. Tried to put it in neutral and it dies out. It will start up again when unplug from gas. Any ideas?????????? It is a 1998 Johnson outboard 40 Horse.