Announcement

Collapse
No announcement yet.

40mm I series carb tuning

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • 40mm I series carb tuning

    When adjusting the high speed screws, for each 1/8 turn out from
    closed, what would be the approximate increase in jet size.
    Assume it is starting with a 145 main. Trying to get this thing dialed in before summer is gone and without smoking the engine.
    It has an after-market pipe on it. Trying to get it to turn the most it can with the installed parts I'm using. Would appreciate your
    expert knowledge !

  • #2
    I haven't a clue.......

    That is not how we go about jetting a motor.

    I was told by the technical dept at Mikuni that you can add up to 50% more fuel by opening the screw 2.5 turns, and that anything beyond 2.5 turns does nothing.

    Honestly, I find this very hard to believe, based on the many siezed motors I've seen over the years..... And, if this WERE true, there wouldn't be a need to re-jet at all.....
    Bill O'Neal WCM
    <a href="http://www.watercraftmagic.com" target="_blank">www.watercraftmagic.com</a>

    Comment


    • #3
      By your description about the Mikuni tech dept........I'll bet that
      NONE of them ever tried to re-jet a carb on a REAL engine.
      Sounds like that was a "theory" statement ! Anyhow, could
      you tell me how YOU go about re-jetting ? I have started one
      size up on Mag and two on PTO. Plugs look (to me) to be a
      little rich. Rpm is getting slightly higher. About 7000 +/- 30. I
      don't have much help around here with aftermarket tuning.
      I looked at the plug reading pictures that Bryan has. Mine
      look a lighter brown than the one he says looks "about right".
      I know that it is impossible to tune over the monitor, but any
      guidance you can provide would be helpful to me.

      Comment


      • #4
        You contradict yourself with the statements that you think it looks rich and at the same time it gains rpm. An overly rich motor loses rpm, not gains rpm. If you add fuel and gain rpm, the motor is telling you it needs more fuel.

        A really safe plug looks brown and has a bit of wetness to it, at least around the outer edges of the metal casing part below the threads.

        Tune and jet until it is so rich that it loses rpm or "fourstrokes", then you have a baseline point that you know is too rich. From there lean it for maximum rpm without ANY loss of rpm on a long WOT run. If you lose just 10 to 20 rpm, richen it up a bit. Note: Do this with a fully warmed up motor. Any motor will lose some rpm after it warms up.

        When all is said and done, remove the head and look at the chambers. They should have a film of oil completely covering the chambers. The piston tops should be a nice brown color, after a wot run. The haed and clyinders should allow you to put your hand on them without burning yourself, and you should be able to hold the sparkplug in your hand without it burning you.
        Bill O'Neal WCM
        <a href="http://www.watercraftmagic.com" target="_blank">www.watercraftmagic.com</a>

        Comment


        • #5
          The reason I said that it looked rich was that the end of the plug (beyond the threads) looked black and sooted, not brown and slightly damp. However, the rpm seemed to be pretty stable at about 7000.

          I also changed the midpipe jet several times without changing the carbs at all. When the water was increased the rpm rose. It also felt stronger in the upper rpm range. How do you know when the amount of water at midpipe is correct with the carb jetting ? I didn't think that changing the water would affect the rpm like that. How do you know what to use as a starting point for the water ? Do you find that the pipe manufacturers water and carb jet settings as a general rule are accurate ? When you rejet the carbs, do you leave the high speed screws in the same place as before or back them out ? I saw in another thread that you stated that if your screws are more than 1/2 turn out you need to rejet.

          I am trying to learn as much as I can about these engines. It takes a while to get through the "learning curve" on tuning these. I'm just trying do this a skillfully as possible without doing too much damage in the process. Your help is greatly appreciated !

          [ June 14, 2002, 10:07 AM: Message edited by: AllPerceptions ]

          Comment


          • #6
            Changing from one pipe jet size to another does not effect the air/fuel ratio provided by the correct jetting in the carbs. Get the carb jetting right, then worry about how much or how little water your pipe needs to run at it's maximum rpm without burning the couplers.
            Bill O'Neal WCM
            <a href="http://www.watercraftmagic.com" target="_blank">www.watercraftmagic.com</a>

            Comment


            • #7
              Thanks again for your help [img]smile.gif[/img]

              [ June 15, 2002, 09:48 AM: Message edited by: AllPerceptions ]

              Comment

              Working...
              X