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Mikuni Jets for Rocky Mtn High

Discussion in 'Norton Commando Motorcycles (Classic)' started by rockitdoc, Nov 8, 2019.

  1. rockitdoc


    Nov 5, 2014
    I know this subject has been discussed a lot, but I'm up in the clouds and couldn't find anything relevant to high altitude jetting.

    I just put a Mikuni 34 on my 850 stock Commando. It came with a 230 main jet and 6DH3 needle installed with the clip on the needle in the middle (#3) position. I'm at 5280' MSL. The kit also came with 220, 240 and 260 main jets.

    What do you gents up here advise? Try it and see?


  2. RoadScholar

    RoadScholar VIP MEMBER

    Dec 28, 2008
    I'd recommend that you use the 220 for a main jet, but the key "players" for the majority of operation are the idle jet, the needle jet and the needle position. I'd recommend trying a #35 idle jet, but having a #30 handy. I'd recommend a 159 O-8 for the needle jet. Start with the needle clip in position 3, but try it in position 2 for comparison.

    When you get a good transition from idle to needle, and on upwards you can advance the timing 3 degrees and see if that puts some spring in your 850's step, do look for any signs of lean.

    If you are not familiar with the interdependence of the idle air screw and the slid height and how to manage the transition from idle circuit to needle metering then please seek the help of a local Mikuni tuner.

  3. eskasteve

    eskasteve VIP MEMBER

    Sep 24, 2012
    You could always give Matt at CNW a ring as he is probably familiar with your areas jetting requirements. I bought my Mikuni setup from him and one of the pre sale questions from him was at what altitude range did I mostly use my bike. It came pre-jetted and was perfect.
  4. seattle##gs


    Oct 28, 2014
    If you are only temporarily at high altitudes such as crossing a mountain pass it is the easiest to just carry a few sets of spark plugs and change them as soon as the others foul out which might be as little as one day. I used to live in Lake Tahoe at 6500 feet. I would use the next hottest plug and change to the correct plug when leaving the lake and going to a lower altitude. That saved me a lot of fiddling. It was interesting to feel the Norton's power begin to drop off as I climbed the grade.
  5. 1up3down


    Jan 12, 2011
    I am in Albuquerque at 6000 feet altitude and I run the same jetting in my 850 Mikuni 34 as you do and I would not change anything.

    Yes the case can be made that we are a little bit rich in settings but so what.
    I have experimented with dropping the needle to be a little leaner in mid range
    but I felt the motor was not as responsive so moved it back to the middle.
    As far as the main jet, I spend very little time at anywhere close to wide open
    throttle so felt no need to lean it out.
    Last edited: Nov 10, 2019
  6. RoadScholar

    RoadScholar VIP MEMBER

    Dec 28, 2008
    Jet for where you ride most. If you change altitude frequently then jet for the average. An idle jet can span a lot of altitude by where you set the idle air-trim screw. A #35 would be a good starting point and the needle clip position gives you options for managing the transition from idle circuit to needle control. Having a spare set of plugs is always a good idea.

    Ambient temperature plays a noticeable role in mixture atomization. Depending on when/where you ride carburetor tuning can easily become a "moving target". Whether I'm on one of my motorcycles with Mikuni, Amal or Dell'Orto carbs I always have a small plain blade screw driver with me.

    Craig likes this.

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