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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a 2002 Mercury 15hp four stroke outboard. It idles

Customer Question

I have a 2002 Mercury 15hp four stroke outboard. It idles and runs fine but is missing at full throttle as if it is being starved a bit. The plugs were very lt. brown. Looks a little lean to me. Motor is basically new. Purchased from a man who used it a couple times only.
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is ***** ***** I will be assisting you today.
How old is the fuel you are using? Did you buy the fuel tank with the engine or are you using your fuel tank from a previous engine.
When you were running the engine did you happen to squeeze the primer bulb?
Customer: replied 1 year ago.
I ran new gas and rinsed the existing tank that came with the motor. The choke was nearly frozen so I freed that up and used some carb cleaner on the carb. It idles perfect and runs up through about 5/8 to 3/4 throttle than starts missing. I am thinking possible main jet obstruction. The motor maybe has 10 hours on it so I don't think the air/fuel mixture has been changed from the factory setting. Plugs look a little lean but that may be the way they set them up at the factory. I squeezed the ball as I accelereated the motor and the ball stayed firm. Thanks for your help.
Expert:  Tan replied 1 year ago.
Ok the jet adjustments won't affect top end. The jets are for idle only.
The high speed jets are in the bottom of the carb and are fixed jets.
Now did you try choking the engine when it was acting up?
Customer: replied 1 year ago.
I pulled the choke out slightly as I ran up the rpms until it started to miss. No help and then to much choke flooded it out. When I romp on the throttle it will go right up with no hesitation but when I steadily increase the throttle it will start to miss at 3/4 rpm. Thanks again.
Expert:  Tan replied 1 year ago.
Ok we need to test the ignition spark at full throttle. This will require the use of a timing light to see if we have a bad ignition coil or another ignition component failing.
We have eliminated the fuel system by squeezing the primer bulb. Then the carburetor by choking the engine. The primer bulb would have let us know if you had a fuel starvation or fuel pump issue. The choke eliminates the carburetor. So that leaves spark issue.
What you need to do is connect the timing ligh to the top cylinder run the engine till it acts up. Watch the flash of the timing light it should be staeady with no dead spots.
If you find a misfire then we need to swap the coil around. Switch the coil wires then swap the spark plug wires.
Customer: replied 1 year ago.
OK thanks. I will have to purchase a timing light and check it out.

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