1999 Evinrude Fitch 150 How to decarbon engine?

1999 Evinrude Fitch 150 on 295 PE Stratos. How to decarbon engine? My rude 150 has never been decarbonized, runs good mid to full throttle. After idling or running slow (no wake zones) the motor wants to stall. Before season all fuel was drained, new plugs indexed and replaced. New fuel filters (2 on boat). Ran fine for fist month then started to stall at idle. Hard restart black smoke from exhaust. Some miss fire as you start on plane until plugs cleared out then it ran good. Any ideas??? only 60 hours on motor, use on small lake and use trolling motor the most. Also checked and no codes in system.

Only 60 hours on a 13 year old motor??? The internals of the fuel system and injectors must be a mess from all that sitting. Mix up a batch of of a couple gallons of fresh fuel, a pint of Yamaha Ring Free or Evinrude Carbon Guard, 3 oz. 2+4, spray in a 1/2 can of engine tuner and then ad a bit of engine oil. Run the motor all at speeds for a 15 minutes minutes then let it set overnight. Repeat this several times to help break up any varnish deposits inside the injectors and fuel pumps. After that, replace the vapor separator tank fuel filter that is pressed into the fuel return fitting. The part number is listed in the 2011 E-TEC parts catalog.

Then clean and regap or replace the sparkplugs and change engine spin on fuel filter.