You need to post up exactly what you have here. Is it the boat in your information? A 1988 Sundowner? Is it an OMC drive? If so your problem could be the ESA sticking ON when the drive is in gear. This can happen if the shift cable is out of adjustment or if the micro switches wear to the point where they stick. The ESA module lowers the engine idle rpm from about 600 down to 450 rpm when you move the shift lever to shift from in gear to neutral. There are two switches:
1) the interrupt switch, this has a little lever on it, and when it gets triggered it lowers the idle when shifting to neutral
2) the overstroke switch, this one is supposed to lock out the interrupt switch so it does not lower the idle when the drive is in gear. However in my experience it can still happen despite that.
The mechanism on the shift bracket senses the load on the shift cable jacket when you try to shift to neutral because the clutch dogs in the lower unit normally do not want to release when there is a load on them. This resistance on the cable jacket causes a lever to move slightly and this triggers the interrupt switch to lower the idle down to 450. Then the drive will go into neutral, the load is off the shift cable jacket and the lever releases and the idle goes back up to 600. If the button on the switch gets worn, it can stick ON. I had his problem on my boat as well about 6/7 years ago. At the time I replaced both ESA switches and the ESA module and had no other problems with it, the original parts lasted approx 25 years so not so bad eh?
so take a good look at those switches.
https://www.dropbox.com/s/ppbuyb06mv2f0 ... c.jpg?dl=0yours might look a bit different but this is the basic set up. If you need new switches or a new ESA module you can get them from CDI electronics in Huntsville Alabama. I got my ESA from them.