Welcome Guest - Sign Up today
Welcome Guest - Sign Up today
Store Links Mobile - Shop Now



Help Tip: If you have a question that has not been answered to your satisfaction in the archives, it is always best to start a new thread of your own. By starting your own thread, you will receive the maximum number of views by forum members.

Below are some additional forum policies in hopes of all iboats members will follow, Thank you.

1. Please do not reply to old topics or hijack existing topics. Old topics of a technical nature are like a library book, Please do not write in them.

2. Old topics should be considered archives and used for reference only. Please do not reply to them.

3. Do not take over someone elseís topic (aka hijack) with your own question, even if it is similar. If you have a question that has not been covered to your satisfaction in the archives, it is always best to start a new topic of your own.

4. If you have a question for the original poster (OP) and the topic is over 30 days old, send the OP a PM, he may not even visit the forums any longer, or may not notice your question in the old topic.

5. By starting your own topic, you will receive the maximum number of views by forum helpers that may not even notice your question when itís posted at the end of someone elseís topic. And those answers will be specific to your particular issue.

6. Please do not post to topics that have been inactive for more than 3 months unless you are the original poster. We have very active forums and any topic that remains inactive for that long should be considered "dead". It is especially confusing when there is an entirely new question posted to an old topic.

7. Posting at the end of any topic is considered to be hijacking the original posters topic which in turn subjects the topic to be closed if it continues to happen thus not making it fair to the original poster in the future had for some reason he/she needed to return for additional information or provide an update of the problem solved which is always welcomed within a reasonable amount of time frame.

8. Please note that you should see a red banner pop up near the bottom of each inactive topic asking you not to reply to old topics. The Red banner will read: Please note this topic has been inactive for 90 days. For the best results, please start a new topic.

Thank you all in advance for doing your part in helping iboats run a smooth ship.

Additional forum rules linked below.
See more
See less

Carb adjustments 3.9 hp Mercury 1968 model

  • Filter
  • Time
  • Show
Clear All
new posts

  • 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.


    • #3
      Sign up today
      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.