Yamaha F75 (2008) loses power

[STDAVIDSBOY]

My 2008 Yamaha F75 four stroke outboard is misbehaving. It starts up and idles fine. It will wind up fine and nearly get on plane but at reaching about 50% of WOT the RPMs drop, it struggles and burbles a little, misfires a little, and bogs down to a little above idle. Once it hits a certain slower speed it sounds like it runs great and there’s a little kick of power above idle at a slow speed maybe 10% of WOT.

I changed the fuel filter after fresh Premium gas was provided. Changed all 4 plugs, which looked a little dark but not unequally dark or fouled. Little water was coming from the cooling drain/exit. I Have a 30HP Yamaha 2-stroke 3-cylinder that would do this bogging down with a bad impeller so the impeller was changed and it was in rough shape having splintered apart on a few of the fins. Plug #1 seems to be fouled up after only a few minutes of use changing the above items. Cool was swapped from another engine. Ran the same way as with the old coil. Plug #1 still fouls. Tried alternate fresh fuel source and line as well. Same result.

What might cause it to lose power?
what should I logically test next?
Where does the shattered impeller plastic go? I don’t run it dry, ever. Why’d it break? Will it have wrecked something downstream?

thanks for advice.
marc



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[rejesterd]

Hi Marc - It sounds like you might have a stuck open injector, which could be causing the fast fouling on cylinder #1. Ideally, you should check the fuel pressure when the issue happens. That's not always easy to do, and you might not have a gauge with the right adapter. Aside from measuring the fuel pressure, you can try removing the injectors and cleaning them (or send them to be cleaned professionally, which is better). You can also try switching the #1 injector with one of the others. Then replace or clean the plug for cylinder #1, re-install it, then see if the fast fouling moves to another cylinder. If it moves, then you know the injector is the problem. If cylinder #1 still fouls faster than the others, then you have to focus on that cylinder. Check its compression and compare it to the others. But hopefully, you'll find that it's due to fuel delivery, not internal engine damage.

Another easy thing to do is simply drain the VST and see what comes out. If you see some solid bits in there or the fuel looks/smells like anything other than fresh, then you might need to replace the filter inside the VST.

As for the impeller, the pieces will go up the water pipe and get lodged (usually somewhere in the upper casing). I haven't had it happen to me, so I don't have much advice on how to get the pieces out. Your guess would be as good as mine there. But even though you didn't run it dry, it still gets dry when the boat is out of the water in storage. Saltwater and age will also contribute to its disintegration.

[99yam40]

do you have gauges to tell you if it is overheating or low oil pressure?
those pieces end up inside the cooling passages plugging up some causing no cooling to some areas.

water pump is a maintenance item that needs to be replaced every couple of years.
rubber gets old and not flexible as it should be

plus you may have had a restriction on the water intake while running, sea weed or other debris in the water

I would pull the stats and PRV if it has one and flush the motor as much as possible to try to get all the pieces out.
may have to remove the heads and exhaust cover if your motor keeps overheating, if that is what is happening