# 2 Stroke Yamaha bogging down, sometimes



## SteveWilson

This is driving me crazy, so wanted to post here in case someone has experienced similar. I have a video illustrating the problem. The motor (2003 Yamaha 115 2 stroke) starts and idles, no problem. I can then run wide open most of the time but then once or twice a trip, it gets into the start shown in the video. No matter how much throttle I give it, it doesn't go above 3500 - 3600 rpms. It will run at that rate for as long as I need to. You'll see at the 1 minute mark how it struggles to plane out when in this state.

To get back to normal performance, I can just stop and restart the engine. Don't need to wait, just stop and immediately restart and I am back to normal. You'll see me do that at the 1:10 mark of the video and I am back to rocket-like performance.




I already replaced the fuel-water separator.
It SEEMs to be a fuel issue so first question, anything else I should consider?
Assuming it is a fuel issue, I keep hearing about the VST filter but I honestly can't find the stupid thing on this motor. Is that a culprit and anyone have a good video or DIY thread?
What about the fuel pump? That is easy to get to, could this be the problem?

Thanks in advance, this is driving me crazy and too sporadic to take to an expert right now!
Steve


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## Tilly_Copano

Had this problem on a 70 2 stroke except I couldn't even get on plane before it bogged down. Cleaned the carbs and replaced fuel pump, good as new.


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## Guest

Tell us more about the motor. It almost seems electrical to me because you can shutdown and restart and she’ll run. Fuel demand is high on acceleration so one would assume as fast as you restarted and took off the problem would still be there?


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## Guest

Also, I am pretty sure it is a carbureted motor so it will not have a vst.


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## SteveWilson

Yes, it is carbureted and that must be why I can't find the VST . Yeah, I don't even have to let it sit, just restart immediately and it seems to reset something. Any idea how to start troubleshooting electric? What could it be?


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## Guest

I can walk you through diagnostics if you pm me your serial/model numbers so I can get you the specs needed. Are you competent with a multi meter?


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## SteveWilson

Yes I am, thanks a lot, will PM now.


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## ARP418

I had similar issue w my 30. Turned out the fuel filter on inside portion of motor had a pinhole leak and was loosing pressure. New print fixed problem


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## ARP418

*O Ring


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## DONAGHUE

Tep sensor or pressure sensor starting to fail??


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## SteveWilson

I thought fuel first too, but the restarting seems to "reset" the motor so it does start to feel like a sensor or electrical of some kind. Starting the process of elimination on my next trip.


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## DuckNut

It acts like it goes into limp mode. 

Don't mind me, beyond my wheelhouse, I have no idea how to help you other than suggesting you haul it over to BB's house. (While you are there you could get some spy shots of the "flower bed") shhhh


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## jboriol

Anti siphon valve sticking on top of fuel tank. Ethanol especially rusts the spring valve and they intermittently stick. Coast guard requires it in the event the boat flips so has does not leak from tank. If you have access to the top you can remove and replace. Its the 90 elbow brass valve that connects fuel line to top of tank. It’s inexpensive and fixed a similar issue I had. 

The VST tank is possible they can rust and pit then clog jets if fuel injection and a that’s much more expensive issue. 

Good luck


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## SteveWilson

Update in case someone else is searching with a similar issue. After much diagnosis, finally gave up and found the right mechanic. Carburetors cleaned and rebuilt and it runs great again. If you have a bogging down issue, try bumping the choke quickly repeatedly. If it revs up again, gains RPMs, it is most likely the carburetors.

I was about to sink her out of frustration, but I like her again now...


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