Older chrysler 55

Looking for suggestions, I borrowed a boat from a family member and it has a older Chrysler 55 hp motor, boat has been sitting a few years so I emptied all fuel, blew out fuel lines, opened carbs and cleaned thoroughly, replaced plugs, replaced battery. got the engine to fire and run good, but every once in a while the motor will not start, I hit it with a little starting fluid will start some times but not always. I checked and it is getting fuel and checked plugs and it is getting spark. someone mentioned that it maybe look like it is getting spark but maybe not enough? is there a way to check voltage at the spark plugs? or does anyone have some suggestions? any help would be greatly appreciated. seams to be an electrical issue but not completely sure?

Make something up to force the coils to jump a gap of 5/16" or more.----If you see and can hear the snap then you have spark.-Testing with a sparkplug laying on a block is simply not an adequate test.

I will try that, THANKS!

Set the carb at 1 and 1/8th turn out and find a manual.The library has them .