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Old 22-09-2012, 16:42   #1
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9.9 Merc two stroke won't accelerate

Worked fine this morning and then bogged down and quit. Started back up and ran enough to get home, but it won't get past 2000 rpm. Changed plugs and cleaned the filter per the manual. I'm betting it's fuel related and may change the fuel line tomorrow. It's eight years old, on a RIB and fresh water kept. Any ideas will be appreciated.
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Old 22-09-2012, 16:53   #2
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Re: 9.9 Merc two stroke won't accelerate

Does sound like fuel, check the float bowl , and the needle valve, might be something lodged in there restricting fuel flow. Sometimes a light tap helps .

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Old 22-09-2012, 16:55   #3
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Re: 9.9 Merc two stroke won't accelerate

Does it run faster with the choke out a little bit? If so then you have to take apart the carb and clean it very well and reassemble. If you are using ethanol fuel then the problem is letting the fuel stay in the carb and it may have gunked up enough to turn into yellow gunk. I run my outboard dry each time I use it by unplugging the tank or shutting the fuel line lever off and let it run until it has sucked all the fuel out of the fuel line.
Start with opening the vent on the fuel tank and making certain your tank is breathing. The next thing is to make certain fuel is getting from your tank to the carb ok and then take the bowl off the carb and see if the float is working properly (if it has a float).
Good luck in troubleshooting and I agree that it is fuel related.
kind regards,
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Old 22-09-2012, 17:44   #4
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Re: 9.9 Merc two stroke won't accelerate

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Originally Posted by SkiprJohn View Post
Does it run faster with the choke out a little bit? If so then you have to take apart the carb and clean it very well and reassemble. If you are using ethanol fuel then the problem is letting the fuel stay in the carb and it may have gunked up enough to turn into yellow gunk. I run my outboard dry each time I use it by unplugging the tank or shutting the fuel line lever off and let it run until it has sucked all the fuel out of the fuel line.
Start with opening the vent on the fuel tank and making certain your tank is breathing. The next thing is to make certain fuel is getting from your tank to the carb ok and then take the bowl off the carb and see if the float is working properly (if it has a float).
Good luck in troubleshooting and I agree that it is fuel related.
kind regards,


In that order.
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Old 23-09-2012, 10:19   #5
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Re: 9.9 Merc two stroke won't accelerate

Therapy, you were right. It did run better with the choke out - I never would have thought of that. It looks like a carb rebuild is coming up!
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Old 23-09-2012, 10:30   #6
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Re: 9.9 Merc two stroke won't accelerate

Make sure your not useing alcohol added fuel ! it's death on 2 strokes !! Just a thought for when ya get it running right !!
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Old 23-09-2012, 10:37   #7
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Re: 9.9 Merc two stroke won't accelerate

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Therapy, you were right. It did run better with the choke out - I never would have thought of that. It looks like a carb rebuild is coming up!

Without being there I would think that the high speed jet is clogged. How does it idle? Usually it is the low speed jet (smaller) that clogs and it runs OK but stalls at idle.

Either way I have not had to "rebuild" for clogged jets.

Just take them out (if possible) and soak them in carb cleaner or "Sea Foam".

A couple of times when the symptoms were mild all I do is load a 60cc syringe with Sea Foam and squirt it into the air intake with the motor running - till it stalls. Let sit 30 min. Start up and see if that was enough. Warning - It smokes A LOT and neighbors will think your garage is on fire.

This works if it is build up of the junk that accumulates from evaporation. If your fuel lines are breaking down then there are little rubber chunks in the jets. These must be coaxed out using soaking and compressed air. Then of course change the fuel lines.
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