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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10621
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Have a 1987 Johnson 150 HP starts ok but at open throttle will

Customer Question

Have a 1987 Johnson 150 HP starts ok but at open throttle will not reach max speed only runs at very low speed
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is***** need to start out with a spark and compression test. The spark must jump a 7/16" gap on a spark gap tester. The compressiomn must be 80 + psi on all cylinders with no more than 15% difference. Now there are a couple of tests you can do on the water to eliminate the fuel system. When you get up to as fast as the boat will run choke it. Does it run better?The other test is to squeeze the primer bulb when engine as up to as fast as it will go. Does it run better?Do these tests and checks and post back with findings.
Customer: replied 1 year ago.
MuchBetter with squeezing bulb no improvement with choke
Expert:  Tan replied 1 year ago.
Ok one more test. Can you run the engine on another fuel source? This will eliminate the boat side fuel system. If the engine runs better on the other fuel source. Then you have a fuel restriction in the boat side fuel system. This could be a anti siphon valve, clogged vent or debris in the fuel tank. If no change then you have a bad fuel pump. The fuel/VRO pump can be rebuilt on the fuel side.Post back with findings. Thank you...Tan

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