Yamaha 90 2-stroke, bogging WOT with video.

[pstephens46]

PROBLEM RESOLVED. Replaced fuel pump and engine runs WOT for as long as I demand; no hesitation.

Replace all filters on a regular basis.

[99yam40]

that was too simple.

glad it was easy

[RABillstein]

PROBLEM RESOLVED. Replaced fuel pump and engine runs WOT for as long as I demand; no hesitation.

[RABillstein]

When you pump the primer bulb you are acting like the fuel pump...supplying enough more fuel to the carbs to maintain WOT. I would think that if you had a fuel tank venting problem the primer bulb would collapse.

My guess is that either you are sucking air somewhere between the bulb and the carbs OR your fuel pump is getting weak.

Fuel pumps are easily rebuilt and/or replaced...at not much cost. Seeing that it is a 2007 engine and new to you, renewing the fuel pump would be a good idea anyway. See how it runs then.

I have been doing my homework and all signs are pointing to a weakening fuel pump. I will be putting a new one on and should be able to get it out on the water this week. As far as filters. they were serviced and replaced by a Yamaha mechanic a couple months prior to me buying the boat. I haven't replaced them myself yet, but I have no reason to suspect them contributing to my issue at this time.

[panasonic]

Have you changed the engine fuel filter and the water separator filter since you had the boat?

[panasonic]

I did everything I could to rule out a fuel tank vent blockage. I ran the boat today and still had the issue despite the gas cap being unscrewed. I did notice that if I squeeze the primer bulb while experiencing the issue, it resolves and returns to WOT. Does this mean the issue can be narrowed down to somewhere between the bulb and the engine (rather than between the fuel tank and bulb)? Should I be thinking fuel pump maybe?

Also to note: I made sure to note WOT RPM at ideal trim (the aforementioned 4500 was without any trim). My WOT was closer to 5300 with speeds in the upper 30s.

When you pump the primer bulb you are acting like the fuel pump...supplying enough more fuel to the carbs to maintain WOT. I would think that if you had a fuel tank venting problem the primer bulb would collapse.

My guess is that either you are sucking air somewhere between the bulb and the carbs OR your fuel pump is getting weak.

Fuel pumps are easily rebuilt and/or replaced...at not much cost. Seeing that it is a 2007 engine and new to you, renewing the fuel pump would be a good idea anyway. See how it runs then.

[99yam40]

since pumping primer helps, I would think sucking air at a fitting or filter maybe

monitoring fuel pressure at output of pump might help troubleshoot.
at least it would tell if the pressure was staying up or dropping while the problem occurs

[Ishvania]

It could be the fuel pump. Its not expensive and its easy to replace.

[RABillstein]

I did everything I could to rule out a fuel tank vent blockage. I ran the boat today and still had the issue despite the gas cap being unscrewed. I did notice that if I squeeze the primer bulb while experiencing the issue, it resolves and returns to WOT. Does this mean the issue can be narrowed down to somewhere between the bulb and the engine (rather than between the fuel tank and bulb)? Should I be thinking fuel pump maybe?

Also to note: I made sure to note WOT RPM at ideal trim (the aforementioned 4500 was without any trim). My WOT was closer to 5300 with speeds in the upper 30s.

[TownsendsFJR1300]

Plus one on both the fuel pump and vent..

It seems to be a fueling issue or lack there of..

[panasonic]

Also try pumping the primer bulb when this is happening. Fuel pump may be weak and can't supply the required fuel at WOT.
Check the venting first as already stated.

[99yam40]

robert beat me to it

[robert graham]

Try running it with the fuel cap slightly loose to allow for venting.....

[RABillstein]

Is your fuel tank venting properly?....Has the fuel pump ever been replaced? My motor is a 1999 C90 similar to yours and fuel venting and delivery are the 2 first things I'd check
first.....

I though about the tank vent, but it fills with ease, and there is no "hiss sound" when I unscrew the tank after its been running. So I assumed the vent was fine- is there a better test for this? The boat is a Scout Costa and the vent it built into the fill neck. VpxaQWTPReqm6Izz7lkmFQ.jpg

I don't know if the fuel pump has been replaced, the boat is new to me.

[99yam40]

change the plugs to rule them out