See full version: Yamaha 90 hp 2 stroke running rough
It does sound like a fuel delivery issue. Like others suggested, check all of your fuel connections for leaks and replace the primer bulb if it is older. Robert suggests replacing the fuel pump, it is not an expensive part. Maybe $40 or 50 as Robert mentions.
I would also start running Seafoam or Cemtool thru the system to clean out the fuel system. You may have stuff in the carbs, old gas. ect. These additives will help with marginal clogs.. A high dose in few gallons would be a good plan, triple does in 3 gallons to start. Put the rest on your tank A can of seafoam treats about 20 gallons.
-Water pump kit with new impeller
-New thermostat
What I know about the motor - it's had around 3-4 weeks occassional use each summer since new so I'm estimating around 400 hours use in it's lifetime. Home servicing has been carried out but I doubt much more than gear oil, spark plugs and the odd impeller change has been done. According to the previous owner a compression check was done recenlty and she has 'good' compression on all 3. After a few hours of use this past weekend I have a few concerns that I'm break down into bullet points to simplify:
New to the forum here and new to ownership of a Yamaha 1990 model 90hp 2 stroke triple which I believe is called a model 90AETO (maybe TRL in USA/Canada). I'm hoping some users here can give me some pointers as to where to look for repair/troubleshooting. I should add that I'm experienced with 2 stroke and 4 stroke motorcycles and have carried out engine rebuilds, carb tear downs and rebuilds on single and multicylinder bikes so I'm looking to do any necessary work myself and beleive this motor to be quite simple in terms of design and maintenance.
-Fuel pump replacement. It seems there is a pump diaphram issue that can cause a misfire on cyliner #3 as fuel gets sucked into the case on this cylinder causing over rich mixture
-Carb rebuild kit - worth doing anyway on an old motor as low speed jets could be blocked more
2. Weak stream from cooling water outlet. The stream isn't that weak but I've had a 40hp Yamaha before and that has a much stronger stream. Also when I touch the jet the water exiting does not seem that warm even after running for 15 minutes. The water temp here (in Norway) is very cold around 12 degress Celcius so maybe this is why it doesn't feel that warm, but I'm suspicious of the Thermostat sticking in the open position. I propose to replace the following: more
3. Engine not fully revving out. At WOT she's topping out around 4900rpm and I can nudge this to 5000rpm with some engine trim adjustment. I was under the impression that the motor should be reaching 5500rpm at WOT. This I imagine has something to do with poor fueling and possible also the prop type/pitch. The boat I'm running this on is 16FT fibreglass deep V hull and weighs in at around 750 lbs excluding the motor. I beleive the standard Yamaha prop is still fitted to the motor. I also noticed that on full acceleration from standing start to full speed the prop is cavitating a little which requires throttling back before throttling on again to regain 'traction'. I'm a little uncertain about how prop sizing works so maybe someone could give me a simple explanation of this? I would have thought a cavitating prop means a prop that is pitched to accelerate fast but wouldn't that also mean that I would max out the RPMs? Ideally I'm looking to trade a little acceleration for a higher top speed and raise the RPMs at WOT. Can anyone recommend a propeller size/pitch for the specs I've given. Speed wise I'm getting around 36-37 knots up top but I'm looking for 40-42 knots that is reported by other owners of the same boat/motor setup.
We went through all kinds of headaches until we called the mechanic. Pin hole in the aluminum head, under load/pressure, would inject water directly into a cylinder. Over time it was enough to bog motor, kind of just showed up. here
It sounds like what was described above as "the thing under the flywheel" is to advance the timing.. like that arm you moved and it smoothed out. If it is and it isn't connected to the throttle linkage it may be a mechanical advance type of thing. like in your distributor in your car. weights and centrifugal force advance the timing as the motor rpm changes. [links]
How did you rule out the fuel pump? When was the last time it was rebuilt? more
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