Are you sure it's ignition and not fuel? Not much to go on.....55 hp Merc 3 cyl. Do you have 3 separate trigger circuits whereby you could verify a cylinder if functioning and then move that circuit to another cylinder and test it there and on to the third cyl.? Idle has no fuel requirement to speak of. WOT needs all the supply system can give it. Bad fuel, cracked fuel lines, dirty filter possibly contaminated by pieces of deteriorated fuel lines, dirty carbs all can contribute to poor running at higher rpms.
On fire I found a battery operated automotive timing light handy. Hook the clip to a sparkplug lead and put the engine through it's paces. If it maintains fire on all cylinders go after your fuel system. If not, chase the one that isn't firing properly. Resistance checks are a place to start, but a high voltage problem in your ignition high voltage circuit(s) only shows it's colors when high voltage is present....aka engine running.
