Yamaha F115 '05 4 stroke random stalling problem

[Scott0301]

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.

Scott

Note: this is a repost because original did not post properly

[99yam40]

If you have to pump primer to get it to restart the you are dealing with a fuel supply issue.
Monitor fuel pressures/vacuum to see what is going on

[patbalmos]

check your low pressure fuel pump I had the same thing happen to mine turns out the spring broke and would sometimes make the diaphram work and sometimes not.

[Triton II]

How about swapping the fuel lines over (if possible)? If the problem swaps engines you have a fuel supply issue on one line.

TII

[Scott0301]

I had pretty much focused in on the LP fuel pump. Thought i'd just replace it until i found out the cheesy little diaphragm pump costs $550. Guess I'd better make sure that's the problem first. Hard to do when the engine won't act up when I'm testing it. I may try the fuel line swap idea.

Thanks for the ideas.