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Old 01-12-2011, 16:55   #1
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Mercury 9.9hp Outboard Won't Idle

My 2006 9.9 Mercury has run fine until all of a sudden the other day it wouldn't stay running at idle and started puffing smoke out the little port which sticks out the back (crankcase breather??). It get progressively worse as the motor warms up and I have to keep it high RPM to stay running. Has anyone experienced this before?
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Old 02-12-2011, 11:03   #2
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Re: Mercury 9.9 Outboard Wont Idle

clean the carb
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Old 02-12-2011, 12:02   #3
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Re: Mercury 9.9 Outboard Wont Idle

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clean the carb
Ditto! The new E10 (10% alcohol) does wonders on carbs if it sit around too long. Inside the carb there is a jet. Take a wire off a wire brush and clean out the jet. Most jets can be unscrewed if that's easier. Check the bowl too for corrosion and clean the float needle/seat with a Q-tip. Alcohol retains water in solution.

For storage run all the fuel out of the system w/o the tank installed.
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Old 02-12-2011, 12:14   #4
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Re: Mercury 9.9hp Outboard Won't Idle

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Has anyone experienced this before?
Yes, we pretty much all have. The sudden inability of the engine to idle indicates that old fuel has been allowed to sit in the carb.

Easy to fix, but even easier to prevent.
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Old 02-12-2011, 13:48   #5
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Re: Mercury 9.9hp Outboard Won't Idle

Yep. That's probably the problem. And if you take the entire bloody carb all the way apart (as I did), you may find it only has a barely visible shellack-like coating on it. I bathed it in carb cleaner and scrubbed the heck out of all parts with a toothbrush, After I cleaned it, though, the motor ran fine.

The shellack seems to take only about a week or so to start forming with ethanol gas sitting in the motor.
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Old 02-12-2011, 14:59   #6
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Re: Mercury 9.9hp Outboard Won't Idle

Thanks all, Now I have to Psyche myself up for possible disassembly... visions of tiny parts running away and hiding :O
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Old 02-12-2011, 16:36   #7
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Re: Mercury 9.9hp Outboard Won't Idle

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Originally Posted by Bay Sailor View Post
Thanks all, Now I have to Psyche myself up for possible disassembly... visions of tiny parts running away and hiding :O
Just don't do the work over the water...
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Old 02-12-2011, 17:05   #8
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Re: Mercury 9.9hp Outboard Won't Idle

The 9.8 has a reed valve assembly in a frame just behind the carb. The early ones had rubber gaskets that sealed this. The new ones have a vulcanized rubber coating as the early units failed and allowed air to leak in without going through the carb.

If it is not the carb (Check this first,.... always) it may be the reed asseembly. good luck
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Old 02-12-2011, 17:05   #9
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Re: Mercury 9.9hp Outboard Won't Idle

My garage...almost as risky
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Old 02-11-2015, 12:39   #10
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Re: Mercury 9.9hp Outboard Won't Idle

I have a 2000 15 hp Mercury and had similar problems, only my wouldn't run unless it was fully choked. Bought a rebuild kit from Mercury Direct, Marshall , TX and rebuilt the carb and it runs great. You can get the part number off their web site. It will show a full blow up of the carb. Only run non ethanol gas, and when you can't get that use Premium gas, it doesn't have as much ethanol in it. Run the engine out of gas each time you use it. I assume it is a 2 stroke engine. Don't let it set up with ethanol gas, the corn settles to the bottom and clogs the carb. Also check your fuel line and if it is older and you have been running ethanol gas in it in the past few years it will eat the inside lining and clog your fuel line. These are all the things I've learned in the past 2 years and it has cost me over $600 to get it fixed.
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