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1999 Johnson 15hp problems

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  • 1999 Johnson 15hp problems

    Motors been sitting for a few years. Finally got it out to use again and I'm having a issue. I'm a atv mechanic so good with small motors. This motor ran perfect when put up so I pulled carb before even trying it to clean. It wasn't too bad, got it all cleaned completely. Then pulled fuel pump to check diaphragm , it was good. I did order a new diaphragm but this one still functions. Hooked everything up and motor will not start unless I pull choke, it hits off and dies, turn choke off and hold throttle wide open and then it will start and sometimes I can keep it running for up to 5 seconds at higher rpms but mostly just bogs out and dies. Then it won't start again till pull choke again. Bowl stays full, bulb stays primed and pump spits tons of fuel when disconnected from carb. I'm at a loss. I did pull the jet completely out just to see and I could make it run wide open throttle but motor just barely running. Gonna get some new plugs tomorrow but they are throwing good spark. Compression checked 100psi both cylinders. I've torn this carb apart prolly 20 times this weekend and it's spotless. I'll try to get a video of what it's doing tomorrow if plugs don't fix it. Any ideas would be awesome. Thanks

  • #2
    Update. New plugs, no fix. New fuel pump diaphragm, slightly better. Mixture screw backed most of way out, it would idle at half choke but die out at any throttle input. Finally got it idling at half choke and starting spraying carb with starting fluid and it cleaned up and would idle and rev. So guess I'm ordering carb gaskets and I'll update once I get them

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    • #3
      Ok, to start with, two stroke motors have to have oil mixed with fuel, or damage will occur. If you plan to spray anything in carb try gas and oil mixture. Carb cleaner, and staying fluid will wash oils away and only damage your motor.

      Secondly, I don't think you cleaned your carb and soaked it well enough, or blew passages out with compressed air, after soaking.

      Third, it is possible the jets, needle valve + seat, and air passages may still have debris blocking some of the fuel, or air flow.

      Also, the air mixture screw should be set at what the manual recommends.

      Good luck, let us know how it's going.

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      • #4
        As I stated, am a atv mechanic. I know the starting fluid isn't good for it and didn't spray it down the carb, I guess I didn't realize I forgot to say that I was just spraying it on the outside to check for air leaks. Carb is 100% clean. Pulled completely apart and cleaned and blew out. I ordered the carb gaskets this morning so once those come in I'll change out. I'm certain that's the problem now.
        With the screw set where book says it won't run at all. It has to be backed all the way out to the point the spring is loose, then thigh ten 1/4 turn. But with the air leak I'm done fighting it until I get those gaskets. I'll update once I get those in but I'm sure it gonna fix it.

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        • #5
          Ok will wait, sorry. Didn't know how you used it, or that there were such leaks. You are probably right on leaks, and gaskets. Post back after work done and let us know.

          Good luck.

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          • #6
            Ok carb kit in, better but not right. It will idle and rev with half choke but as soon as you kill choke she bogs out and dies. Screw is 3 turns out. I can keep it running with choke off but it's only at higher rpms and as soon as it starts coming down it dies out and no amount of throttle will bring it back. I'm gonna just leave it and put it in water this weekend and see if any change or not in water. I'm stumped as to where to go from here tho.

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            • #7
              Ok well I tried something just for shits and giggles. Saw someone post to not run NGK plugs, so I pulled the 2 brand new NGK's I installed and replaced them with the original 1999 Champion plugs from when the motor was new. Yes the 15 year old plugs and now it runs perfect. No lie, starts up and idles, revs up and doesn't die even with choke off. I'm gonna go pick up some new Champion plugs and try those. Doesn't make any sense to me but I'll try her in the water this weekend or maybe even this week if I get a chance.

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