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Yamaha 90 2-stroke, bogging WOT with video.

RABillstein

New member
My 2007 Yamaha 90TLR will run perfectly for the first few miles on the water. Once warmed up, if I leave it at WOT for a little while the engine bounces back and forth between a lower and higher RPM. It sounds like I am blipping the throttle, and it slows the boat down until I back off the throttle then slowly reaccelerate. It will run WOT again for a little while then do it again. It does not give me the issue at lower RPM, only at WOT and just below WOT. Only seems to do it after running for a while; not for the first five-ten minuets after leaving the dock. I have not been noticing any alarms. Seems to do it no mater the engine trim angle.

Curious if any of y'all have had the same issue or know what the issue might be. Sounds like it could be fuel delivery related, but I don't want to guess just throw random new parts at it.

Also, any recommended Yamaha mechanics between St. Augustine and Daytona Beach? Looking for a tech that does good work, and is honest. Might have to go that route if I cant figure out the issue myself.

Sorry for the vertical video. Video starts with the engine "bouncing". It self corrects, then starts bouncing again around 17 seconds.
https://youtu.be/_rHz0BGFzn0
 
Yes had this exact problem I suspect it was fuel pump or possibly the air vent screw on fuel tank not letting air in. Try squeezing primer bulb when driving.
 
4500 at WOT seems low, even without the bogging.

I would assume it's a fuel-related issue. That's good you don't want to throw parts at it, but I would start by inspecting things in the fuel system.

I don't suspect a leak in the fuel lines because the issue would still occur in the first few minutes after you're away from the dock.

The mechanical fuel pump can be disassembled and inspected. Could be a diaphragm leak or a check valve failure.

The fuel filter cup should be emptied into a clear container, so you can see how much debris is in there (and also if there's any water). Then replace the filter. This really isn't 'throwing a part at it' because this is something that should be replaced regularly anyway.

After that, there's just the carburetors themselves. You can try fast-idling and holding a rag over each carb intake (after removing the intake silencer) for a few seconds. This can unclog the jets sometimes. But probably best to take them apart though if you can.
 
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.
 
My money is on fuel pump order a pump rebuild kit plus the little oval gasket that goes beteeen motor and pump. If its not the pump its a good thing to service anyway. Might as well do filter while your at it if just unscrew the clear bowl by the pump there is a little mesh filter inside.
 
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?

There could still be a leak between the bulb and the tank fitting. But it does confirm that the problem is fuel related. So check the easy things.. fuel pump, and fuel filter (the one on the engine). If you're sure the pump is working properly and the filter is clean, then move onto the carburetors.
 
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