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Jeremy R.
Jeremy R., MarineTechnician
Category: Boat
Satisfied Customers: 9019
Experience:  Multiple factory certifications 20+ years of experience.
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I have a 2003 4-stroke Yamaha outboard that has been checked

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I have a 2003 4-stroke Yamaha outboard that has been checked for compression and all four cylinders checked exactly 90psi. Have replaced the fuel pump, added planer boards to engine, it wont plane out with one or two good size adults in the boat. It seems to have no push out of the hole. We got it to finally plane out with one person on the bow and it ran about 6200 RPMs at approximately 30 knots. But it still has no out of the hole power. What could we check next?
Hi, and thank you for letting me try and assist you with your problem.

90 psi is way too low for a 4 stroke motor.
The fact that you say all cylinders were exactly 90 leads me to believe that you didn't have the throttle valves open, or the guage you're using is broken.
every motor has varied compression between cylinders.
You just need them to be with in 5% to 10% of each other.
First thing you should check is timing.
Mud Dobbers are notorious for building mud nests on the timing belt and as soon as you crank it up. The belt will jump 1 or 2 teeth and can cause this exact problem.
Customer: replied 5 years ago.
Spark was checked and I do not believe the carbs were synchronized just adjusted by sound no electrical gages were used. I will check for the mud dobbers but the belt looks clean. Where can I get a compression gage myself to be sure and fit these spark plugs they are different than an autos?
They compression gauges at an auto store will work.
Carbs out of sync will cause poor idle but not what you're experiencing.
I'd say if you're compression and timing turns out to be good you need to have the carbs ultrasonic cleaned.
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