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Hi folks!
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.
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:
1. Rough Running: Start up from cold is relatively easy - the auto choke function does not seem to operate yet it starts regardless with a touch of fast idle added on the control unit (ambient temp is around 60-65 degF). Idle seems a little rough with occassional misfires but idle holds around 900 rpm and no stalling/cut out. Low speed running (1200 - 2400rpm) in forward gear is very rough, sounds like its running on 2 cylinders only and I need to throttle up to 75% throttle to clear this and as soon as she hits around 3200 rpm she signs nicely on all 3 cylinders while accelerating at this throttle setting. Once on the plane and throttling back to around 50% to hold anywhere below and up to 3900 rpm she's running better but still rough, I can hear unsettled running and slight misfires at this speed. I need to run the engine at 4,000 rpm and above to achieve smooth normal operation. From what I have read I aim to look at the following:
-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
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:
-Water pump kit with new impeller
-New thermostat
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.
I realise I've suggested fixes to a few of these issues myself here but it would be great to get some input from more experienced forum users to assist me!
Thanks is advance folks!
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.
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:
1. Rough Running: Start up from cold is relatively easy - the auto choke function does not seem to operate yet it starts regardless with a touch of fast idle added on the control unit (ambient temp is around 60-65 degF). Idle seems a little rough with occassional misfires but idle holds around 900 rpm and no stalling/cut out. Low speed running (1200 - 2400rpm) in forward gear is very rough, sounds like its running on 2 cylinders only and I need to throttle up to 75% throttle to clear this and as soon as she hits around 3200 rpm she signs nicely on all 3 cylinders while accelerating at this throttle setting. Once on the plane and throttling back to around 50% to hold anywhere below and up to 3900 rpm she's running better but still rough, I can hear unsettled running and slight misfires at this speed. I need to run the engine at 4,000 rpm and above to achieve smooth normal operation. From what I have read I aim to look at the following:
-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
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:
-Water pump kit with new impeller
-New thermostat
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.
I realise I've suggested fixes to a few of these issues myself here but it would be great to get some input from more experienced forum users to assist me!
Thanks is advance folks!
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