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2002 Merc motor runs rough at slow speed

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I purchased a new merc motor 40 H.P. 2cyl. 2stroke in June of this year. The first time out it flooded out badly. I took it back to the dealer and they adj. the float level. The next time out it did not flood out anymore but it idled very rough on slow speeds. Took it back to dealer where they again adj. the carb and charged me 85.00 because warranty does not cover carb adjustment. Now today after two trips to my dealer with maybe 2 or 3 hrs. on the motor it still runs rough when trolling. The motor starts very good and runs good at high speeds any suggestions would be appreciated.

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Bring it back to your dealer and tell them to inspect the screw that holds the float pin.
We had the same problem with one of our 40's.
Half way around, on the under side of the screw head, it is beveled, if the screw is not positioned right the float will hang.

We would get a tank or less of gas through our motor befor it acted up, after 2 trips to the shop and 3 attemps by us we finally figured it out.
Haven't had a problem since.
Good luck


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I'm no motor expert. But I know that on the newer, bigger motors, the engine doesn't run on all cylinders when operating at low rpm. I'm not sure what you mean by running rough. But MY dads motor operates in the fashion that I described above. and when you hit the gas, you can tell when it kicks into all cylinders. Maybe this is your issue but probably not.

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how old is your gas??? on the first few tanks you were supposed to mix the gas a little rich then after those tanks you were suppose to change plugs! just a couple of ideas. good luck

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You might be trying to get performance too soon. If you only have 2 to 3 hours on the outboard, you haven't broken it in. It's a good idea to run at least a full 6-gallon tank of gas thru the engine before you run it for extended periods at steady speed.

As stated above, use a bit heavier mix, run a full tank thru, then troll. Avoid wide open throttle for first 15 hrs.

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