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My Yammaha F150 has been losing between 800 and 1000 rpms at WOT... It will run perfectly for a few minutes when first started, then lose the rpms and stay that way.... I changed the water/fuel separator (no water in it), cleaned and changed the fuel bowl and filter on the motor (again, no water or trash) and changed the plugs.... it still lost the rpms, so I took it to the shop... the mechanic changed the plugs (again) and thought it was fixed... NOPE...so took it back again...this time we took it out for a spin while the computer was connected to the motor and he recorded the loss of rpms and a skip at about 2000 rpms but the computer still showed absolutely nothing wrong with the motor so he suspected that there was possibly a colapsed fuel line somewhere... NOPE.. not that either...so now they are sending the injectors to be cleaned.... I'm still not sure the injectors are causing the problem, but what else could it be? My questions are: Should the injectors need cleaning on a motor with less than 350 hours? Could ethanol fuel clog the injectors, and if so, why didn't the computer show the injectors to be bad? and if the problem is the injecotrs, why does the motor run perfectly for a while before it loses the rpms?
I'm stumped.

Not sure if this is your problem but worth checking,, newer boats are required to have an air vent connected to fuel tank, these have a check valve inline. make sure its working. maybe try connecting a portable tank to motor and give this a try. will eliminate any on board fuel problems.
Had similar issue and mechanic from taylor co. replaced check valve and wha-la problem resolved..

Thanks, they claimed they checked the fuel lines and fuel pressure but didn't mention anything about checking a valve... I'll have to ask them... after doing some internet searches, I'm leaning toward the problem being a fouled oxygen sensor... but I really don't have a clue and I don't think the mechanic does either.

Here is an idea, try a new mechanic. Seems like he is guessing and you are paying for his education.

I had the same problem on my f115 yamy and turned out to be carbon build up. Maybe worth checking

It is possible that there is a spark problem. It will not always show up in a diagnostic at that rpm and load. Have you tried plug wires? Coil?
FF

The resistance in a wire/coil/relay/sensor is breaking down once the engine starts to warm up.

I suspected a coil / spark problem too, so I pulled each plug wire from the coil while the engine was running.. each broke down as it should have when pulled, and the mechanic says that the coils (both of them) test fine. When I changed the plugs I nooticed a slightly different color on one so I'm thinking it has to be a coil, or a fuel problem and since the coils test good, I'm guessing fuel or the oxygen sensor. ?????????????????????????????????????????

Maybe you power pack has gone berserk. Hope not.

Well, after a week and a half and a couple trial and errors by the mechanic, I got the boat back today... It was an expensive lesson ($1200.) but I learned a couple things I thought I'd pass along...

1. 4 Strokes don't have oxygen sensors

2. If your outboard has injectors... get them cleaned periodically especially if your motor starts skipping or losing rpms...

3. If you own a Yamaha and have it computer checked.... it won't show clogged injectors

The problem was 3 out of 4 injectors were pretty badly blocked and weren't spraying correctly. The cost of cleaning the injectors was $200.. the rest was labor.... THAT won't happen again!!!

4 stroke car engines have oxygen sensors, why wouldn't boat motors? Are you sure?

Use SeaFoam on a regular basis and sue you over-priced mechanic for negligence and over-charging.

leonreno wrote:4 stroke car engines have oxygen sensors, why wouldn't boat motors? Are you sure?

Nope, not sure at all.... that's just what the mechanic said when I told them to check it out.

Could you tell me who that mechanic is? I just dont want to step foot in his shop!!

Williams Boat Crossing..... Dothan AL