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Setting up carburator for high elevation

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  • Setting up carburator for high elevation

    Hello, Nate here. I’ve been creeping these forums for 3-4 years and now I’m finally getting around to do some work on my 2004 1700 Roadstar. It has BuB pipes that look like all the guts have been taken out and a kuryakyn high five air cleaner kinda looks like the alley cat series. I live at about 4,200 feet above sea level. I need to pull and clean my carb and while I’m at it I’d like to do all carb mods in the forum. My question is would the jet set up listed in the forums 172/40 jets 4th ring up on the Barons needle still work at my elevation? Some of the guys here are telling me they run with 163 or 162.5? Also there many articles on tuning and adjusting the carb but no articles that I have found take you from start to finish on what to adjust first before going on to the next step of adjustment. Oh and I’ll be porting my intake while I have the carb out and doing the fuel pump delete by putting in the grizzly float needle. Any help would be much appreciated.

  • #2
    That is the beauty of this carb, no need to worry about elevation changes, Jet it for the 1700cc and your good.

    Comment


    • #3
      That’s great news! Thank you for the fast response roadiemort. I’ll order the parts right now.

      Comment


      • texasscott1
        texasscott1 commented
        Editing a comment
        I think the Mikuni advice is for their manual slide carburetors. Our stock carburetors automatically compensate for altitude.

      • TimB
        TimB commented
        Editing a comment
        LOL Believe what you like Scott, but you are wrong. It's easy to verify.

      • texasscott1
        texasscott1 commented
        Editing a comment
        Tim, I have verified it. I've ridden from here (1300 ft) to Denver and Cheyenne without any problems at all and there are those who have ridden up to 10,000 feet with just the expected loss of power because of the lower atmospheric pressure. The lower pressure doesn't allow as much fuel to be pushed up through the jets and also the diaphragm doesn't raise the slide as far for a given throttle position.

        The manual slide carburetors will allow the slide to be raised higher and result in a richer mixture at higher altitudes even to the point of black smoke.

        The stock carburetor on the Road Star is supposed to be able to handle altitudes up to 10,000 feet without any modifications. And I've never seen anything printed by Mikuni stating otherwise.

    • #4
      From the Four Corners area, northwest New Mexico.

      I believe that the 172 jet will be a little rich for 4200' elevation. I would drop two jet sizes and the needle on the forth clip if it were me.
      Make sure you set your float after putting the Grizzly float valve in. I set the fuel level at 2mm above the bowl mating line on the carb exit side. Since your going pumpless, make sure the fuel tank is vented good and filters on the Petcock are clean. I sell Petcocks that all of the ports are opened up, new gasket kit installed, and threaded exit nipple.
      Adjusting the PMS screw is fairly easy. Start with the PMS screw turned out 2 1/2 to 3 turns, bike warmed up and the idle is around 950, then turn the PMS screw in or out to find the highest idle. Once you find the highest idle, turn the screw out 1/8 of a turn. If you have to turn out more than 3 1/2 turns, then change pilot to one step richer. Less than 1 1/2 turns out, pilot one step leaner. This is the way I do it.

      Comment


      • #5
        If you are running the Stock 40 CV Mikuni. Don't worry about altitude.. It is a CV, Constant Velocity, carb. The same jets are installed at the factory whether it is going to Miami or Denver and they come out of the crate running fine. They compensate for the change in Barometric pressure. . If it is an old style flat slide carb which some folks change them to, it needs to be jetted for altitude. No offence guys but you try to complicate things to much with these. CV carbs..Very easy to tune for your BAK, . Best thing next to FI.

        Comment


        • #6
          Don't forget that when you change the air box to a better flowing box, along with free flowing pipes, that it will change the dynamics of what the CV carb will do. CV carbs are great, but with the change in air flow "not air density or air pressure", then it requires a jet change. You can make the CV come alive if you do it right.
          If you don't understand how it works, then don't screw with it.

          Comment


          • #7
            Beauty of the jet kits for our stock CV carbs, is that they come with fine instructions... every one I've rejetted worked perfectly fine, as per the instructions, as soon as I fired the bikes up!

            Comment


            • TimB
              TimB commented
              Editing a comment
              Never bought a "jet kit", I use only Mikuni parts.

            • Questcap
              Questcap commented
              Editing a comment
              I've only ever snagged jet kits off of eBay years ago, when you could actually buy one for under $30! Yea, it's been at least that long!

          • #8
            Click image for larger version  Name:	IMG_0954.JPG Views:	0 Size:	101.2 KB ID:	66720 You know I thought the subject of Altitude jetting was well know for awhile. When I joined the clinic years back I was ridiculed and had insults thrown at me when I stated that the CV would compensate for Altitude...I have rode from home at 1200 feet, over the Rockies The High Sierras. and all mountains states west of the Mississippi. In the past 40 years. No problems. I have stayed in a cabin located at 7000 feet for a month and rode the area. The thought of rejetting never entered my mind. The only adjustment for altitude was to increase the idle., it will lower at high altitude. Yes a normal aspirated engine will lose HP as altitude increases... 3% per 1000 ft. It is not because it is running rich on your CV carb.. It is actually running leaner. Density altitude will present the same effect. I know most do not even know what I am talking about. I am no self proclaimed Guru. Just a well trained aircraft mechanic who happens to know about carbs and altitude effect on different types.. My MPG goes from 40 to 45 at home to as high as 60 MPG at high altitude depending on how hard i ride it. It goes for my Roadstar my Venture my Goldwings , My Shadow, what ever I am riding with the CV... I won't comment on this anymore the facts are the facts. Or as they say the proof is in the puddin!! Texas Scott 1 you are right..

            Comment


            • TimB
              TimB commented
              Editing a comment
              LOL Big difference in riding through & living somewhere. I ride from 11,000 to 600 & if you can't the difference in the way your bike runs, you aren't very observant

          • #9
            You bet I can tell,.. Loss of power but not from jetting. I am damn observant. You get that way from flying over 20k hours... Don't LOL me. I know my business.

            Comment


            • #10
              Ok fellas, here is where I am at. I did everything as described above. 172/40, 4th clip, ported intake, blah blah blah. It is indeed running just a bit rich but way way better than before. I’m using the old school method of tuning per the post somewhere here where you run your finger the last 2-3 inches of tail pipe. I smell just a hint of fuel and my finger prints are black but not caked on. Ifollow up with my final set up ends up being.

              ps....after posting this I realized I put my Jesus clip on the 4th from bottom... I’ll report back if this was the issue. If not I’ll order one main jet size lower.

              Comment


              • #11
                Don't forget that you will get carbon on your finger if you put it in the tailpipe. Hell it is an internal combustion engine,They all do that. Now go stick your finger in the tailpipe of your car that has O2 sensor and computer controlled fuel management. Your finger will also come out with black carbon on it.

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