I recently sold my 1986 Bomber Scout to a young man. The last time I ran the boat was last October and it was fine, and I did an earmuff test for him in my driveway, where I cranked it and let it run in neutral for 5 minutes or so.

 

The problem is it will crank fine, idle fine, and will cruise around 6mph fine, but when he tries to go on plane the engine will cut out. He can get it to idle with high RPMs fine, but as soon as he kicks it into forward gear and tries to get higher RPMs it will cut out.

 

He replaced the spark plugs, fresh 87 octane gas with sea foam, gas line bulb is firm, engine light is green when it shuts down. Any ideas on what it could be?

Carbs, it seems to always the the carbs. I watch too many outboard repair videos even though my motor is brand new. This guy repairs a ton of outboards to flip em. https://www.youtube.com/c/ThatBoatGuy/videos

 

For your case specifically,

 

I’ve bought a few motors that ran just like that in my day. Run great in a barrel of water and bog down under a load. One guy gave me my cash back and the other guy conveniently stopped answering the phone. The guy that gave me my money back said the thought maybe one of the cylinders was getting a lean mixture 

I've been messing with things a long time, buying, fixing and selling.  The first two things I do to any motor that has sat for a while and not used regularly, is pull the carbs off and clean/rebuild them, and drop the lower unit and put a new water pump in it.  

Even if it does seem to run OK, both of those items can fry a motor in no time.

If the motor was regularly used, and carbs look great, I still drop the lower unit and replace the water pump impeller.  That's one of the most neglected, critical items on a boat motor.  I change them every three years, but I had 15 and 20 year old motors that had never had one changed.  

That like playing with dynamite,  and not knowing when it's going to blow and when it does, it's usually very expensive to recover from it.  I forgot to mention, if the fuel pump is a model that can be rebuilt, you should clean it also, the valves can be sticking.

 

By the way, for those that say they put SeaFoam in one to clean it out, after the fact.  SeaFoam will do almost nothing to clean a gummed up dirty carbs.   Then you have those that say they sprayed carb cleaner in them, totally useless.   The ONLY way to clean the carbs is to pull them off and take them apart.  

If that is done by someone that knows what they are doing, most likely the motor will run great.