My engine has developed an intermittent misfire or stumble at wot,it gives you the sensation that you are running out of gas I assumed it to be a fuel delevery problem so I pulled the carbs, they were clean, jets were clean also. Fuel pump diaphragm looks like brand new and pump holds air pressure (5 psi) with no leaks, when underway and problem developes primer bulb isn't collapsed so fuel line to tank is ok and fuel filter is clean. Removed plugs and they seemed good. I'm left to wonder if this is now an ignition problem? I've had this engine for 15 years and it has preformed flawlessly up till now so any help on this matter would be appreciated. Its an 87 Yamaha 40 hp tiller, electric start oil injected, operated in freshwater exclusively.