2000 C90TLRY Wet Plugs & Poor Economy

[gander]

My 90 Hp (premix fuel only)seems to use a lot of fuel. The engine runs fine on bottom and top end. I pulled the plugs after the engine had been run and they were all wet with fuel. I thought the electrothermal valve might be sticking open, but it is getting voltage with the engine running and passed the tests recommened in the Clymer manual. Any help would be greatly appreciated. BTW, is the manual you recommend the Yamaha factory manual? Thanks again for the help.

[Ken]

gander,
Were you previously getting better fuel economy and suddenly now you are not? Are you able to get max rpm as you were before? Is motor starting easily and idling smoothly? What do you mean by "electrothermal valve"? The choke?
Poor fuel economy usually is accompanied by other symptoms like loss of hp and max rpm, hard starting, and rough idle and low end rpm. You might want to check the choke on that 90 - take the air breather off and look at the butterfly choke valve to make sure it is not stuck partially closed. You can manually move it with the manual pull choke knob thru its range - also, look at the butterfly when you engage the electric choke to make sure it opens all the way back when choke switch is released.
If you can give me some answers to the above questions, maybe I can help you some more.
Good luck,
Ken K

[gander]

Hi Ken,
I bought the boat and motor used and have had it out two times. The second time out I used about 9 gallons of fuel and ran about 35 miles, a bit less than four mpg. The motor is on an 18ft. Alumacraft center console, so I don't run the motor wide open. I have power to spare. It's a flat bottom and starts to porpoise around 40mph. I cruise at around 38mph comfortably with the engine turning around 4,000 rpm. I have no reason to believe that the engine wouldn't run wide open just fine. Starting and both cold and hot idle are fine. Comes out of the hole like a jackrabbit.

This engine is equipped with the electrothermal valve(no choke butterflies) which is open when the engine is cold and puts raw fuel to the engine when it is cold. As the engine warms up the valve closes and the rich supply is cut off.

Today I pulled and checked the thermostat. I thought that it might be stuck open causing the engine to run cold but it checked out fine, not stuck open and opened and closed right on spec. when heated in hot water.

Again, all three plugs are wet with fuel, leading me to believe that I'm running too rich.

Thanks for taking the time to help.

[Ken]

gander,
Ok,now I am with you - I think that choke system is also referred to as "prime start".
Your mileage is not too shabby - I'm only getting about three mpg with my C115 on a Pathfinder that weighs only 1100 pounds.
The wet plugs has me baffled - you would think the motor would be missing badly for the plugs to be wet. Maybe that choke system is kicking in when it is not suppose to at low rpms? Can you monitor the voltage to the valve to see when it is activated? After the motor warms up, I would think it should not activate at all.
Yes, I would recommend that you get a Yamaha shop manual for your model outboard. Check the needle valve settings per the manual - maybe yours are set too rich.
Let us know what you find out.
Ken K

[gander]

Ken,
I got my shop manual ordered. Every time I've checked the plugs was after idling. I will check to see if my idle mixture is too rich. I believe only the idle mixture and idle speed is adjustable on this engine, but I'll check it out. I also want to see if I'm propped right. I'll keep you posted.

BTW, does the Seafoam decarbon and also clean the carbs? Where can I buy some?

The Yamaha site lists test results on a boat like mine with a carbureted 2001 Yamaha 90 and the best mileage they got was 6 mpg. I think I can do better than I am.

[Ken]

gander,
Yeah, your prop may be pitched too low - I would guess it should be around 19 inches. You want max rpm around 5000 when on plane and trimmed out.
Doing a shock treatment with Seafoam will decarb the pistons and rings and also clean the fuel system of gum/varnish. It will not do as good a job of cleaning the carbs as removing and disassembling and cleaning them, but if the carbs are not in too bad a shape, Seafoam will clean them up ok. I bought Seafoam here locally at Advance Auto - most auto stores should have it.
Good luck,
Ken K

[Ken]

gander,
I just thought of something else - since you bought the motor used, check to make sure the correct plugs are installed.
Ken K

[gander]

Ken,
Yes, the plugs are what Yamaha recommends. My prop is 13-1/4" X 17 SS.

My manual calls for adjusting the pilot screws between 1 to 1-1/2 turns open from lightly seated. I can't find any explanation of what the screws do. I'm guessing that they adjust the fuel/air idle mixture. Is this right? Mine are opened over two turns out from lightly seated. Could this be causing me to run rich at idle?

[Ken]

gander,
Yep - that could be causing you to be running a little rich at idle - set those needles where the Yamaha manual recommends. And,yes, those needles adjust the fuel/air ratio.
What is the weight of your boat w/o motor? Do you have a tach to give you accurate rpm? You should be able to stop that porpoising by trimming the outboard in a little and then WOT to get max speed and see what your max rpm is. If you go to a 19" pitch prop, you will probably get a little better fuel economy, but your hole shot will decrease a little.
Ken K

[gander]

Ken,
I'd have to guess on the weight. Probably 1650# with the motor. I'm basing that on the test I referred to. I do have a good tach. My next time out I'll trim it in and see what top end RPM is.

I got a new 13-1/4" X 19 aluminum prop and hub kit with the boat so I can give that a try. Trouble is, down here in S. Texas I usually manage to find an oyster reef with my prop. One trip through a reef and the aluminum prop is toast.

[Ken]

gander,
Yeah, I don't know why they put aluminum props on outboards - I hit a finger mullet with mine and bent it - stainless steel is the only way to go.
Keep the aluminum as a spare - that's about all its good for.
If your boat with motor weighs 1650, then your hull weight is about 1350 - that 17" ss prop is probably about right - do the max speed/rpm test - that will tell the story.
Later,
Ken K