90 AETO not revving past 3500

[will101]

Hi any input here will be much appreciated!

My one motor is only revving up to 3500 RPM and I have checked the usual things and replaced the racor fuel filters. I did a visual check on the filter on the outboard but did not see anything suspicious.

Before I take the carbs apart what else can I check? Motors are "new" with less than 50hrs, although they are '99 models. They have been standing a while.

I noticed this weekend at about 2000 RPM, the revs tend to be uneven between 2300 and 1900 RPM, but at idling and open throttle the revs are constant. Took the spark plugs out after flushing (should maybe try again after an open throttle run) and they were all a bit black and oily. none were particularly worse than the rest. The gaps seemed a bit big - did not measure, but does anyone know what the gaps should be?

Sorry for the long-winded blurb, but hopefully I can get some help with this!
thanks and regards
Will

[boats.net]

Sounds like carbs do need to be cleaned, that will probably get it going.
Plug gap is 1.0 mm.

[will101]

Thanks I will attempt to do it myself - hold thumbs!

[will101]

just an update

Problem still not solved but herewith some new info.

Running the motor on its own, it can only get about 1900 rpm under load so clearly the Port motor is doing all the work.

It does not splutter or run uneven and replacing the spark plugs as well made no difference.

Tonight I will run her under load with a portable tank, and if that makes no difference, I have been advised to check the fuel pump diaphragm for damage. THEN open the carbs I guess.

I would still appreciate some general input from other users!

[99yam40]

check your alarms to make sure they are working and you are not in some alarm mode. A manual is something that all motor owners need and will give you a lot of needed info
check fuel pressure with a gauge and use a piece of clear tubing to see if you are sucking air.
If you are starving for fuel you will hurt the motor if you keep running it.
Carbs do need routine maintenance. pull and clean like Yamaha says to
Good luck and keep us posted on what you find.

[will101]

Sorted

Well the problem was found to be a blocked passage in one carb. What I would like to know, is how on earth can one carb (the bottom one, you guessed it) become so completely blocked up and the other still look like absolutely brand new? On the attached pic one can see what the float bowl drain plug looked like. The same "muddy" substance blocked the final stage passage.
Can anyone tell me how this happens or what this material is? Could it have been generated by water in the fuel?

[99yam40]

Good to hear you got it fixed.
The bottom carb seems to always end up with more remaining fuel in it even after trying to run them dry, this fuel and whatever is in it dries up and leaves residue. If you are going to set the motors up for a long while it is best to drain the bowls after fogging motors.
Yamaha recommends pulling and cleaning carbs on routine maintenance along with lots of other things. Hopefully you have a manual by now and will take good care of the motors and get good service out of them for many years to come.