Mukuni 33 jetting starting point

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  • #1133
    Wade Metzinger

    Where would you guys suggest a good starting point for jetting?

    I live in OK so not altitude jut heat and humidity.

    Current jetting:

    Main .059

    Pilot .205

    Main air bleed 8

    Pilot air bleed 6

    2005 Halfscale ZR3 Shockwave 8.90

    Matt Parlett

    So if this is a 33mm Mikuni carb, your main fuel jet is actually the 205 and the pilot fuel jet is the .059.

    When you are looking at the back of the carb after taking off the air filter you will see an air jet in the left and the right side. The jet on the left is your pilot air bleed and the one on the right is your main air bleed.

    For the main air bleed, most people will typically leave it alone and some even block it off completely. For the main circuit we do nearly all of our tuning on the main fuel jet, the one that is currently at a 205. During the spring and fall a 205 may be ok where you are but you would likely need something a little leaner, like a 195 during the summer months.

    Both the pilot air bleed and the pilot fuel jet will get changed on a somewhat regular basis in order to maintain the same starting line temperatures throughout the seasons. The pilot circuit controls all of the air/fuel characteristics when the engine is sitting there idling. So getting the correct air/fuel ratio is essential to getting both a smooth idle and getting the engine temperatures up to a warm enough point.

    Think of the pilot air bleed as the fine tune adjustment and the pilot fuel jet as the coarse tuning adjustment. If you wanted to lean out your pilot circuit which will typically result in a smoother idle and higher temperatures, we would start by leaning out the pilot air bleed one or a couple steps at a time. I typically like to stay within a .7 to a 1.1 air bleed and when I reach one of those edges, I will then make a pilot fuel jet change and also an air bleed change.

    .6 & .59,   .7 & .59,   .8 & .59,   .9 & .59,   1.0 & .59,   1.1 & .59,   .8 & .57,   .9 & .57,   etc…

    To summarize, the pilot circuit controls how smooth your idle is and what tempearutre the engine gets up to on the starting line. The main circuit controls everything after wide open throttle and is responsible for down track performance.

    The only Mikuni tool that I am aware of that can help you with this is our RaceBase software. It has a Mikuni tuning tool that will tell you what jet to change your main jet to in order to maintain the same air/fuel ratio mixture you are used to.

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