03 Yam115 no-load seems fine, load won't go higher than 3500-4000

Our o3 Yam 115 inline 4, 4-stroke seems to run fine in the boatyard, revs up, no problems. In the water, under load, won't go beyond 3500-4000.
Increasing throttle at those speeds even seems to slow the engine a bit. Can get occassional bursts of speed above 5000 but does not stay there. Yam tech analyzed electronics and said everything OK except pulsercoil. After about $400 for one of those, absolutely no difference. Dismantled hi-P fuel pump: seems to run and pump in a bucket of gas ok.
fuel bowl was very clean. Changed out low-P fuel pump and temp sensor. No diff. Swiitched to a different key/safety switch and complete electrical harness: no diff.
Great forum. Thank you for any help you can offer.
Capt. Baum

Re: 03 Yam115 no-load seems fine, load won't go higher than 3500-4000

Capn B, will the engine EVER go above 4,000 RPM? In other words, if you nail the throttle from a dead stop, will the engine go above 4,000 RPM and then drop down to 4,000 and start running rough? If so, then I would guess that it could possibly be a fuel delivery problem. I think there might be an in-line fuel filter between the LP fuel pump and the VST that could possibly clog and cause a restriction. This is a weak guess on my part through - some of the seasoned Technicians on this forum are the right guys to dispense the advice.

Re: 03 Yam115 no-load seems fine, load won't go higher than 3500-4000

monitoring the fuel pressures at problem RPM should tell you if there is a fuel problem

Re: 03 Yam115 no-load seems fine, load won't go higher than 3500-4000

Woodnaut said:

Capn B, will the engine EVER go above 4,000 RPM? In other words, if you nail the throttle from a dead stop, will the engine go above 4,000 RPM and then drop down to 4,000 and start running rough? If so, then I would guess that it could possibly be a fuel delivery problem. I think there might be an in-line fuel filter between the LP fuel pump and the VST that could possibly clog and cause a restriction. This is a weak guess on my part through - some of the seasoned Technicians on this forum are the right guys to dispense the advice.

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Thanks, you guessed right: it will go to higher revs (sometimes) if I quickly advance the throttle, runs there a bit and then falls back to 3500-4000. Have replaced the in-linefilter between the lowP pump and the highP pump and that did nothing. I am going to bypass the main gas tank and run a small auxilliary directly to the in-side of the low pressure pump, placing the small test tank high enough above the pump that the gas should flow by sipon if nothing else.

Re: 03 Yam115 no-load seems fine, load won't go higher than 3500-4000

99yam40 said:

monitoring the fuel pressures at problem RPM should tell you if there is a fuel problem

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Thank you for responding. The shop manual says that the fuel rail pressure at just key on should be around 40 PSI, and engine running around 32. My running pressure seems about right. BUT, the key-on-only pressure seems very low. How can the engine-on pressure be higher than the key-on-only pressure?
Thanks for the help.