9.9HP Mercury Outboard Carburetor Cleaning/rebuild



A look at how the carburetor works on a small outboard, and how to remove, disassemble, clean, and reassemble the carburetor. At the end of the video we will also talk about how to set the air/fuel ratio (mixture) using the idle adjustment screw and mixture adjustment screw.

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24 thoughts on “9.9HP Mercury Outboard Carburetor Cleaning/rebuild

  1. Own a 15hp mercury and last weekend it started great but then stalled out.
    Removed cowl and saw standing gas in the bottom of the housing. Cranked her over and gas poured out the bottom of the carb. This was after I had pumped up the primer bulb to rock hard as fellow at boat ramp suggested. Pulled the start cord again and motor ran briefly but rough.
    I inspected the 2 plugs and both were badly flooded.
    I let it sit for about an hour, pulled the cord and she again ran briefly but rough.
    When I got the boat home I cruised You Tube and suffered through several abysmal videos on carb cleaning – no tripod, horrible sound, terrible lighting – one was even on a boat and was hand held – I almost got sea sick.
    Your video is clean, well organized, great lighting and pretty good sound.
    I think your use of the go pro for closeups was great.
    I followed right along with your video in little chunks and eventually finished my carb overhaul.
    I get to try it tomorrow morning.
    Sam

  2. I also thought the float might be stuck in the open(down) position but that was not the case. I disassembled the carb and cleaned it as per the video – some clogging in the high and low speed jets but gave both a good cleaning.
    I really think pumping the bulb rock hard forced too much gas to flow and stalled out the engine.
    After reinstalling the cleaned carb and two pulls on the rope, she started right up on the third pull and runs like a charm, even at idle.
    I would not have attempted this repair without this video. The first I could get in for a repair was one month away and goodness knows how much money.
    I hope that this mechanic and his team will feel really encouraged do more videos.
    They are providing an amazing service.
    Sam

  3. Great video, well done. I have a 2005 Merc 25hp 4-stroke bigfoot. Usually starts right up when I get to the lake. First or second stop and restart motor bogs down and dies on take off. Then hard to start. Once restarted, usually is fine the rest of the day? When it does get into hard start mode, sometimes takes me 30 minute to finally get restarted. Sometimes I have to drain the bowl. Then have to open the throttle level all the way open until it spits and sputters and finally restarts. Then usually good for the rest of the day. Yesterday I removed the carb and removed the bowl and thoroughly cleaned with carb cleaner. Everything looked in good shape. Took the boat out in the evening. Started right up. I noticed is seemed to run much stronger and full speed. Again though, after the first or second stop, same thing, started but died at take off. This time however, with throttle level opened all the way it spit and sputtered but started fairly quickly. Then it was good the rest of the day. Any ideas?

  4. I'm trying to soak all the relevant parts in carb cleaner dip, but I can't figure out how to remove the choke plate and the plastic turny-thingy that holds it in place. How on earth does one remove this?

  5. My 2014 9.8hp Nissan outboard wants to die idleing. I cleaned the carb and it starts very easily (usually with one pull), but it keeps wanting to die on idle. Could excessive fuel supply be the culprit? Because for some reason, when I disconnect the fuel line, it idles very smoothly and stops sputtering and dying. Or should I mess with something else? Any suggestions would be greatly appreciated. And also, this is a great video. Very well done!

  6. Thanks Rowintel for this outstanding video. It gave me, a novice mechanic, the confidence to dive in and clean the carb on my 8hp 4 Stroke Merc which was running rough or quitting at idle
    and unusable for trolling. The carb was indeed dirty and I cleaned it per your instructions. In the process, I learned how carbs work. After putting it back together the engine started easily and purred like it should. The idle and mixture adjustments shown at the end of the video were invaluable. Fortunately, the brass plug for the mixture adjust screw had already been removed so I didn't have to deal with that drill out. Your background explanation and then step by step explanations exemplify what good how-to videos should be like. Well done and Thank You!

  7. Awesome video bro. You just earned yourself another sub. I love your workplace shit everywhere just how I like it. A wrench here a socket there carb cleaner over there. My wife tells me you need to this place. I reply (with crazy eyes) don't touch anything lol.There's a method in the madness. My problem is my mercury 9.9 won't idle unless I continually press the primer button on the motor. (not the gas tank primer) I've checked the fuel pump it's shooting lots of fuel when disconnected from the carb. Could you give me your thoughts on this. Stuck float maybe?

  8. I own a 15hp, 4 stroke Mercury outboard, 1999. It will not run at idle. I can get it to idle briefly by giving it full choke.
    I thought this indicated not getting gas at idle.
    I followed along with your video and got to where you remove and cleaned both the high speed and the low speed jets. I got the brass high speed jet out, cleaned and replaced but I could not get the low speed jet out – I could get it unscrewed but was unable to get the actual brass jet removed for cleaned. I tried tapping with the carb upside down but the jet would not fall out.
    I gave it a good shot of carb cleaner and will let it sit overnight and try again in the morning.
    Any thoughts why the jet will not come all the way out?
    Sam

  9. Great video! Do you know if the 20hp is much different. Also I can't get the 10mm socket to the back bolt because the air intake is in the way. Any suggestions? thanks

  10. 2006 MERCURY 9.9 4 STROKE – I HAVE BEEN THRU THIS CARB CLEANED THOUGHLY REPLACED THE NEEDLE, BOWL GASKET, CHECK VALVE IN BOWL (IT WAS STUCK), PRIMER DIAPHRAM. I WILL ADJUST THE MIXTURE SCREW TO YOUR SETTINGS. THE PROBLEM IM HAVING IS THAT THE GAS IS FILLING THE OIL RESERVOIR? DRAIN IT AND TRIED AGAIN – SAME THING GAS IN THE OIL. SUGGESTIONS ….THANKS JB

  11. This is the best instructional video I have ever seen on the web. The detail that you go into is essential for a full understanding of the job.
    Your video gave me the knowledge and confidence to clean the carb which turned out to be the source of my problem – very rough and unreliable idle.
    Question – are you a rigger for the University of Michigan?

  12. I have a 2004 mercury 15hp 4 stroke and it will only run when the choke is on. Fuel is getting to the carb and the motor and carb looks very clean, however the brass plug has been drilled out. Before I pull the carb to clean do you have any ideals. Thanks for a great video.
    Ron

  13. Great video, explanations and general commentary. Your comments regarding the purpose of the little "plunger" at 7:19 helped me fix a particular problem that I have had with my 1998 Mariner 15HP 4-stroke since I purchased it new. It would often stall when shifting from neutral into gear. I checked and sure enough the little plunger was not getting activated – so I adjusted the screws and it now purrs out of neutral with NO HESITATION. Thank YOU. No mechanic to date had picked up that the plunger was not getting activated by the throttle linkage.

  14. Any advice on removing the main jet that just "falls out?" Mine is stuck good and will not come out. The low speed jet came out with  a little resistance but otherwise undamaged.

  15. need help my 9.9 4 stroke mercury when you pump the fuel bubble gas come out the overflow hose that is on top of the carb what will make it do that please help

  16. OUTSTANDING, I cleaned the carb reinstalled it and it ran fine. Late in the day I decided to run it again and it wouldn't idle. I took the carb off again and it was clogged again. fuel line and or filter had junk in it..

  17. Oh man, you did more than just save me money, you gave me CONFIDENCE!. I just successfully fixed my 9.8 Evinrude (it's a Tohatsu engine, really) after watching your video. My motor died while out 2 days ago – and because it's spring all the shops are saying it's 3-4 weeks before they can even look at it. I changed out my gas first, tried Seafoam treatment but no luck. So, based on your video, I took the Evinrude carb off (it's pretty much identical). I followed your cleaning directions and re-assembled. Pumped the gas and she started 2nd pull ! And is now purring like a baby. I've never touched a carb in my life until today. THANK YOU.

  18. Greetings from Norway. Great video as usual! Wonder if you could help me on something. I have the 15hp mercury, it's 321cc but they sell it as a 10hp here, and there's a small plastic stop you cut off to increase the rpm. Was wondering if it would be possible to take it to 20hp with maybe a different carb. There are some stores here that say it's possible, when selling on the website. Would appreciate your advice. I was just wondering if increase the rpm would work, because I notice alot of slack on the throttle linkages?

  19. Thanks for the superb instruction – I followed along easily and saw improvement when I accelerate quickly. One thing that remains a struggle on my 4 stroke merc 9.9 is cold starting. Once started, it starts first pull, all day long. It is difficult to start without starting fluid and even more difficult to start without using the throttle only button and speeding up the throttle pretty high. I would really appreciate any suggestions you may have.

  20. Thanks so much! You saved me $200 – and more importantly, you save me two out-of-order weeks during our precious short summer on the water

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