I have a pair of 2005 F115 EFI 4-strokes on my boat, both have 684 hours since new, regular routine maintenace. Both engines start and run nice as can be at all speeds from idle to WOT, except port engine will stall every hour or so, always when running nice and smooth at constant RPM. Once stalled it will not restart, but if I squeeze the primer bulb several times it will restart instantly and run just fine for another hour or two, then usually stall again. Stalls happen at any RPM from idle to WOT. Some days it runs perfectly all day long, like the days I take it in to have the problem diagnosed.
It never stalls when changing throttle position or shifting, only when it has been running for quite a while at a fixed RPM. The stall is not instant, but begins with several seconds of bogging and RPM drop. The engine starts and runs great and develops full RPM and power. Thought the check valve in the primer bulb might be shot, so I replaced the bulb, but no joy. Plugs look good, no junk in the fuel filter. Always use marina gas with no ethanol.
I know lots of things that can result in stalling, but most have other symptoms, e.g., rough running, hard starting, bogging with load changes, low RPM, etc. I've got none of those, just an engine that runs perfectly, but only 99% of the time. It's that darn 1%.
Looking for ideas from the Yamaha sages here.
Note: this is a repost because original did not post properly