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Old 01-07-2015, 17:46   #1
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Merc 2-stroke 9.9 trouble shoot

Friend gave me mid80's 9.9 Merc outboard. Starts with choke out, after 10 seconds or so rpm's go way up. Pushing choke in brings it back to idle, but dies after 10 seconds or so. Pull out the choke just before it dies and the process will keep repeating. Adjusting idle control knob does not help. Seems to have OK spark, has fresh gas - any ideas?
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Old 01-07-2015, 18:16   #2
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Re: Merc 2-stroke 9.9 trouble shoot

Sounds just like a bum carb. For some reason it seems impossible to get one to run right without a new carb or a good ovhl maybe.
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Old 02-07-2015, 04:36   #3
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Re: Merc 2-stroke 9.9 trouble shoot

Greetings and welcome aboard the CF, NorthPole.
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Old 02-07-2015, 04:48   #4
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Re: Merc 2-stroke 9.9 trouble shoot

It may be a bad carb. But before I spent that kind of time and money, I'd clean the tank, fuel lines etc on he suspect idea that the fuel was bad-parts fouled with either water or residue from ethanol tainted gasoline. Bob
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Old 02-07-2015, 09:01   #5
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Re: Merc 2-stroke 9.9 trouble shoot

the jets are gummed up in the carburetor, not really that hard to clean most likely the jet on the bottom after you take the fuel bowl off. look inside the stem and you will see what looks like a plug with a slot for a screw drive and a hole in the middle that hole will be clogged up. you not be able to see it but it is there! the clog google it and watch a you tub
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Old 02-07-2015, 09:41   #6
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Re: Merc 2-stroke 9.9 trouble shoot

I just whent through this with a Johnson 8hp. Just take the carb apart (all of it. I forgot to clean out the low speed jet, that cost me an extra 5 hrs of troubleshooting!!) clean it with carb cleaner until you think its clean, then clean it again. reassemble with new gaskets and it will fire up. at the same time you should install new plugs. The ethanol in most gas really gums up the carbs fast.

Once you get it running I would get into the practise of running it until its dry (disconnect the fuel hose when you're done for the day and run the engine until it runs out of fuel)

2-cycle engines are great, less complicated, less costly and lighter!
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Old 02-07-2015, 10:46   #7
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Re: Merc 2-stroke 9.9 trouble shoot

Mid 80.s must be a 9.8 , know them well . The carb has a blue cover on the side , open it up and make sure the screen is clean . Thats a start, also the gasket between the carb and the engine is crucial to these motors IT MUST BE PERFECT !! make sure it is good and does not block the small holes to the carb , so matter what you do you will have to take off the carb drop the bowl clean it all up , check the operation of the needle valve by blowing into it and lifting up the float , make sure it closes of the fuel supply , clean the jets , replace the gaskets !!!! and you should be good to go .

Regards

O yes new fuel
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Old 02-07-2015, 11:47   #8
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Re: Merc 2-stroke 9.9 trouble shoot

It's the carb. If you don't know how to clean it, now is a good time to learn. Go online to one of those outboard parts for sale websites and print the schematic of your carb. Take it apart, spray down the parts with carb cleaner (I use Chemtool carb cleaner from auto parts house), pull a small wire off of a wire brush and make sure you poke thru all of the holes (especially the small ones on the top that are for air intake). Then put it all back together. I can do this in 20 minutes total. Have done it before on one engine while we were motoring across the bay on the other! Ethanol fuel is hell. I recently flushed and cleaned the entire system, changed filters, fuel lines and bulbs, and went to non-ethanol gas. If you search for it, you can find it. Basically eliminated all of my carb issues.
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Old 02-07-2015, 17:14   #9
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Re: Merc 2-stroke 9.9 trouble shoot

Quote:
Originally Posted by pcmm View Post
I just whent through this with a Johnson 8hp. Just take the carb apart (all of it. I forgot to clean out the low speed jet, that cost me an extra 5 hrs of troubleshooting!!) clean it with carb cleaner until you think its clean, then clean it again. reassemble with new gaskets and it will fire up. at the same time you should install new plugs. The ethanol in most gas really gums up the carbs fast.

Once you get it running I would get into the practise of running it until its dry (disconnect the fuel hose when you're done for the day and run the engine until it runs out of fuel)

2-cycle engines are great, less complicated, less costly and lighter!
I have a Mariner 9.9hp from '96 which had the same sorts of problems. Agree with pcmm...clean carb. You can get good drawings for the motor online. I bought a rebuilt kit, did a thorough fuel system clean out. Now runs like a champ. I also run out my fuel after each days use. Change plugs seasonally. Motor starts 1st or 2nd pull on choke without fail....Great motors those 9.9's...

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Old 02-07-2015, 22:19   #10
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Re: Merc 2-stroke 9.9 trouble shoot

In Guatemala, I had a small business of getting Outboards to run right. Nearly everyone was a dirty carb. One time I had one that would not run right at all. Everytime you started it, and it started easy, It would run for a couple minutes and then quit. I finally found that the liner inside the hose from the accessory tank to the motor was broken and although the gas would run through the hose you could not suck it through fast enough to sustain the motors use. Mac
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Old 02-07-2015, 23:08   #11
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Re: Merc 2-stroke 9.9 trouble shoot

Yes mate, take ot the main jet a get a bristle from a wire brush and poke it through all the little holes while washing constantly in petrol. If you have access to air compressor easy peasy,
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Old 02-07-2015, 23:58   #12
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Re: Merc 2-stroke 9.9 trouble shoot

Hello NorthPole,
Good advice from all. My money is on the idle jet. The clue is that rpm increases just before it dies when you push in the choke. That is b/c the idle jet being blocked causes the fuel mix to be increasingly lean in the few seconds before it starves enough to die. The idle jet is the one with the smallest dia hole, about the dia of a gnats eyelash. As someone suggested, use the wire from a wire brush (or a welding torch picker) to clean it, being very careful not to enlarge it.

Also, before you call it good, be sure tht corrosion on the inside of the fuel tank is not the source of the material blocking the jet. Thoroughly rinse/swill the tank out as best you can with fuel , and each time, suck it empty using a hand transfer pump you can get from marine /auto parts stores. Suck it out via the outlet hose , not the filler cap. If you cant get it clean and dont want to replace the tank, install 2 filters in series (the ones with transparent shells so you can see when they are dirty).

Back to the carb.. it has a spring tensioned, screwdriver adjustable jet, pretty obvious. I wd not unscrew that to clean it, chances are that it is not blocked unless the carb was really fouled. If you dont take it out, you wont have to mess with setting it, can be quite critical and a pain to get just right. It was probably set right by the previous owner/tech. If you do take it out, count the number of turns, accurate to 1/8 turn, so you can restore it to where it was.

To test the engine, it must run without stalling on the lowest speed on the throttle handle, and must start hot or cold.

Oh, did you check for water in the lower unit oil?
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