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Carb adjustments 3.9 hp Mercury 1968 model

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  • Carb adjustments 3.9 hp Mercury 1968 model

    Does anyone here have just the information out of a repair manual perhaps for the correct carb adjustments (screws etc...) for a 3.9 hp Mercury outboard? I'd buy a manual but all i need are the carb settings and the engine goes back to the owner all its costing him is parts i see this as a learning experience for me and he cannot afford alot so thought i'd try to help him out. I did rebuild the fuel pump and had taken the carb apart and cleaned it out. Any assistance greatly appreciated.


  • #2
    Re: Carb adjustments 3.9 hp Mercury 1968 model

    Carb adjustments are all pretty much the same. Set the low speed jet, and setting the highspeed jet if its not fixed. General procedure is this. Bottom out the low speed jet, (gently) turn it out 1 1/2 turns (CCW in most case's) bottom out the high speed jet, (if there is one), turn that one out again 1 1/2 turns. Now start the motor, let it warm up. Adjust the low speed jet, (in neutral) so it will idle. Now if there is a high speed jet, you need to have the motor loaded up, ie either in a tank with a test prop, or on a boat in a lake,river etc. Run the motor up to WOT, slowly adjust the high speed jet out (CCW 1/4 turn out a time, wait a moment to feel the change, then continue.) untill the motor richens up, and starts to miss. Now turn the high speed jet in, (CW) untill the motor starts to rev high, as you keep turning in 1/4 turn at a time, the motor will hit the highest rpm, when you find that point, turn the jet back out ccw 1/4 to 1/2 turn. NOw for the low speed jet, bring the motor down to an idle, again same procedure as for the high speed jet,
    turn the jet out (CCW 1/4 turn at a time, pausing after each turn) untill it stalls out. IF your quick you can turn the jet back and stop it from stalling. Otherwise, turn the jet back 1/4 turn and restart the motor. Now turn the jet in (CW) untill the engine leans out, and stalls. Move the screw back, 1/4 to 1/2 turn, this will be your low speed jet setting. In that range somewhere you will get the motor to reach its lowest idle. That should be close for your motor, however I am not familar with it, so I may have missed something.

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    • #3
      Re: Carb adjustments 3.9 hp Mercury 1968 model

      There is no high speed adjustment and after the basis setting on the idle mixture, the fine tuning on the idle adjustment must be done with a bonnet with a hole in it for adjustment purposes or after the engine is warmed up the engine is stalled out and the bonnet removed and the engine is restarted by spinning the flywheel with a rope or between your hands to restart it for idle mixture and speed adjustment.

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