2008 yamaha 8hp hi thrust

westhill

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May 3, 2025
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3
2008 8hp high thrust. Motor is difficult to start, idles rough. It revs up to high rpms in neutral, but in gear it onlys gets to about half rpms at full throttle. No rpm guage so this is approx. I have cleaned the carb 3 times everything look shinny and clean then i guessed i was doing something wrong so i just bought a new carb. I replaced the plug coil and then plugs, they are gapped to spec, I have replaced the fuel pump and the fuel line and bulb from the motor to the fuel filter before the tank, this motor shares a tank with the big motor, big motor runs perfectly. I also replaced the fuel filter in the motor but not the fuel lines they looked good. I also did not replace the fuel connector on the motor. While running in neutral at high rpms if I move the throttle quickly back and forth the motor will pop(backfire) in the silencer. Timing is correct according to the manual. All it says is line the arrows up. I did notice doing this the both cylinders are at TDC at the same time. Motor was running fine, i loaded the boat trailer home about 2hrs and then trailer to a different lake about 1.5hrs the next morning and when I went to start the motor is was tough starting and wouldn't idle well and wouldn't get to full rpms in gear

Any help with what could be wrong would be appreciated im a fishing guide and this has caused me to cancel trips
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,522
Sounds like it might be running on 1 cylinder.----Does spark jump a gap of at least 5/16" on both leads.----And post the compression values.----Yes , both pistons go to TDC together.----And both fire at the same time.----One being a " wasted spark " when piston is on exhaust stroke.
 

westhill

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May 3, 2025
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3
Also I have checked compression. Top cylinder is 148, bottom cylinder is 153. My only chocking of spark was to hold the plug against the motor and both looked good, blue spark
 

airshot

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Joined
Jul 22, 2008
Messages
5,434
Buying a new carb means nothing, unless it has been setup and adjusted for your particular motor. Many carbs are made to fit a number of different motors, so they need setup to match a particular motor. Have you checked your fuel for water !! If spark is good and carb setup correctly, water sounds like it might be an issue.
 

westhill

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May 3, 2025
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Yes fuel is new and ethanol free, and it goes through a water seperatetor/filter before it goes to the motor. Carb is set to the manual
 

airshot

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Jul 22, 2008
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5,434
I would still test the fuel, most will tell you they have bought new fuel that has water already in it. Good luck....
 
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