90HP Yamaha - No top end. Help with diagnosis.

[Jim Hawkins]

The motor is a Yamaha, 90HP year 2003 (I think) When I first got it several months ago it would do about 30mph at close to 5000 RPM as setup. Recently I have lost top end power. It still runs very smooth at idle and under power, it just does not get above 3500 RPM's and about 23 mph anymore.

I first checked the linkage to the carbs and verified the throttle was moving them the full range with no apparent problems.
I checked compression - 110 PSI in all three cylinders, the same as when I bought the boat a few months back.
Next I considered it may be running on 2 cylinders but placing a hand over the throat of each carb in turn while idling affected all 3 the same.

From here my best guess is to pull and clean the carbs but not wanting to put in that work unless needed and also considering Murphy's law and the "If it ain't broke, don't fix it" idiom I want to be pretty sure thats the best solution.

Any thoughts on pinpointing my issue?
Thanks.

 

[99yam40]

check fuel pump, change spark plugs, drain carb bowls and catch what comes out to see if water or debris is found in any of them.
also fuel goes bad quickly if it has ethanol in it

 

[Jim Hawkins]

check fuel pump, change spark plugs, drain carb bowls and catch what comes out to see if water or debris is found in any of them.
also fuel goes bad quickly if it has ethanol in it

The boat is run a lot and I constantly check for water in fuel/water separator and demijohns used to fill. I recently replaced all fuel line and fuel filter and was careful to flush the new line before connecting. The plugs are almost new and have been cleaned and put back in different cylinders.

I considered a restriction in the fuel line so I put 2 gallons fuel in a portable tank, added SeaFoam and ran the boat back and forth across Cudjoe Bay at full throttle for a few minutes, then stopping and starting over pushing it full throttle out of the hole. It ran flawlessly without a hickup or hesitation, just not higher than 3500 RPM.

My experience is that water, bad fuel or bad plugs would be noticeable.

I will try pumping the primer bulb in that it may show a weak fuel pump.

 

[robert graham]

Have carbs ever been removed/disassembled/cleaned?.... Could be some trash in jets...Mine is a 1999 C90 and is always willing to show me 5500RPM's, but if it ever doesn't I'll be fixing it quick...before a lean carb condition does some serious damage to motor....

 

[Jim Hawkins]

Have carbs ever been removed/disassembled/cleaned?.... Could be some trash in jets...Mine is a 1999 C90 and is always willing to show me 5500RPM's, but if it ever doesn't I'll be fixing it quick...before a lean carb condition does some serious damage to motor....

I have only had this boat 4 months Robert, and no, in that time I have not pulled them and have no idea if they ever were. I'm used to motors with 1 carb and 3 is just so doggone many. I actually enjoy giving a carb the once over when I'm familiar with the motor and it's easy to take off and on. I suppose I was just hoping someone had the same issue and there was a simple adjustment or something. Rats.

 

[99yam40]

get a service manual for your motor if you plan on working on it yourself

 

[robert graham]

If you have a good Yamaha technician nearby, let him clean and set your carbs for you....maybe $150, but a lot cheaper than a burned piston!...

 

[Jim Hawkins]

If you have a good Yamaha technician nearby, let him clean and set your carbs for you....maybe $150, but a lot cheaper than a burned piston!...

Good advice Robert but what I'm thinking is regardless whether I do it or someone else does it I still want to be sure first that it needs it. If I know for sure the carbs are causing the problem I'll clean them and enjoy doing it but if they are not the problem then it's time or money wasted either way. (I know, kind of like insurance. Money wasted if you don't use it but it buys peace of mind)

Just wish there was a well to tell. In the past whenever I had carb issues it was pretty obvious but on this motor everything runs so smoothly starting, out of the hole and up onto plane. It's just as if I have a smaller HP motor now.

 

[99yam40]

make sure fuel pump in not leaking fuel into #3 crankcase

 

[Jim Hawkins]

make sure fuel pump in not leaking fuel into #3 crankcase

I have to admit I have no idea how to look for that. Can you help me?

 

[99yam40]

Have you looked in the infrequently asked questions part of forum?
here is thread on testing fuel system which includes pump
http://forums.iboats.com/forum/engi.../290505-how-to-testing-your-boats-fuel-system

but if you remove the bolt s and pump the primer, if it leaks fuel from the pulse port there is a problem

 

[Jim Hawkins]

make sure fuel pump in not leaking fuel into #3 crankcase

OK, so I have checked the fuel pump for leaks, cleaned and checked gap of the nearly new plugs, run the motor with good fuel in a portable tank to eliminate any questions about the boat side fuel lines/filter etc. I have read over the fuel system thread above and everything points to the carbs.

Also, just to verify, in my experience, losing fuel pressure is usually noticeable by the engine losing rpms as a vacume builds up from the pump trying to pull fuel faster than the line will allow., and air in the lines will be very noticeable by making the engine sputter. In my case the motor runs smooth and does not lose rpm's when it is at full throttle.

Any last advice?

 

[robert graham]

Guess it might have a broken reed valve petal....have to remove carbs to check the reeds, although I don't think it's a common problem.....just an idea for you...

 

[Jim Hawkins]

cover me, I'm going in...