[jaredman2]

I have a 2000 Yamaha 115 4 stroke outboard motor on a 22ft pontoon boat. It runs fine for a distance at max speed then the motor starts to bog down and the rpms go way down. We stop and sit for awhile or just run at a much slower speed and then in a while we are able to run at max speed for a while. It does this repeatedly. I have had it in for repairs numerous times and no one has been able to solve the problem. Any Suggestions? Please.

[haskindm]

I also have a 115 4-stroke but have not had this problem. I had a Mercury outboard that was doing exactly what you are describing. It seems that there is an "anti-siphon" valve on the fuel tank that is supposed to keep the fuel from siphoning out in the event that a fuel line ruptures. It was sticking closed. Replace valve, problem gone. One way to check would be to see if the bulb in your fuel line has collapsed when you are having the problem. If not, if you pump the bulb, does the problem go away briefly?

[Woodnaut]

I had a similar problem with my 2006 F90. In my case it turned out to be partial clogging of the in-line fuel filter between the low pressure fuel pump and the VST. Only a small amount of fuel was flowing. The VST would fill up at idle, and it would run OK below 2,700 RPM, but that was it. The fuel line arrangement on the F115 is probably a little bit different, but the principle is the same - put a restriction ANYWHERE in the line and you can get some - but not enough - fuel flow.

With the engine off, I disconnected the low pressure fuel line where it enters into the VST. In my case, the primer bulb had to be squeezed very hard and then I only got a very small fuel stream. This indicted the restriction was downstream of the primer bulb. But restrictions can occur anywhere in the line from the pick-up line in the tank through the fuel filter(s) and pump(s).

If it does turn out to be a restriction, try and find the cause. In my case, gunk had built up in the filter due to water getting into the fuel tank. I had to replace the filter and eliminate the source of the water entry.

Perhaps a faulty fuel pump could also cause this problem - I'm just not sure. You might also check all fuel line connections from the tank to the fuel pump to make sure none are sucking in air. Hope this helps - Woodnaut

[rodbolt]

the 2000 F115 did not run a filter between the fuel pump and VST, later versions did and its a reccomended upgrade. woodnaut has it about correct only you may have to clean the VST filter as well. its part of routine maint.
are you taking this to an actual Yamaha dealer or to a local for the "numerous" repairs? this isnt a hard problem to solve.
a remote tank can eliminate your hull systems and the EFI fuel rail pressure tester can confirm or eliminate the VST filter.