Rotax Needle Setting

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  • #42827
    Scott Mowrey
    Participant

    Hello all,

    Since it’s been so hot I have not been out running lately, but I do have a question concerning the Rotax carb slide needle. I understand that there are 4-5 settings for this thing, and it’s what you adjust to get more low-end power. Since I can’t seem to get going once my RPMs are down in the 3-4000 range (pulling out of the pits or coming back on to the track after spinning out) and at operating temperature (145-150 degrees) I was told that the needle needs to be adjusted. Thoughts?

    #61834
    Garrick Mitchell
    Participant

    I only know Rotax, but I’ve never found them to be spectacular below, oh, 7k RPM. Of course, all I’ve ever tweaked has been the main jet.

    The needle clip governs how high the needle sits in the carb. There are multiple grooves the clip can sit in on the needle… The higher the groove, the leaner you are at partial throttle. At wide-open throttle, the needle is out of the picture regardless of clip setting, so it doesn’t really do anything for “low-end power.” The Rotax manual actually explains it pretty well (but then again, I’m a Mechanical Engineer, so take that with a grain of salt :loony: ).

    Conventional wisdom is to leave the clip in the highest groove, maybe 2nd highest, but otherwise it impacts operation only occasionally (how long do you keep it at 1/3 throttle?).

    #61835
    Scott Mowrey
    Participant

    Thanks for the feedback Garrick. I’m only asking because the kart bogs down leaving the pits and after a spinout. Also, if I don’t hit the hairpin just right at The Track, I could make a sandwich in the time it takes for the kart to respond to the throttle. I KNOW I could pick up a second or two if I could get this bottom-end thing worked out…

    #61836
    Doug Welch
    Participant

    Put the needle in the 2nd position and leave it alone. That’s the second groove from the top. No Rotax will pull below 6,000. Even then, you can’t mash the gas. You must squeeze the throttle.

    #61837
    Anonymous
    Inactive

    I just leave mine in position #1 and never adjust it

    #61838
    cgordon
    Participant

    A Rotax will always bog after a spin or when you first leave the pits, so don’t worry about that.

    It can vary a bit from engine to engine, but I found that the 2nd clip position was usually best here. The way to check it is to move the needle down to 3rd position then drive on the track and see if the kart accelerates cleanly from slow corners when you roll onto the throttle to accelerate. If it chugs, it’s too rich. If it hesitates then goes, it’s too lean. Go back to the pits and move the needle clip accordingly and repeat the procedure until the kart accelerates cleanly. I always used a K98 needle.

    Make sure your main jet is in the ballpark. It can vary by a jet size or so, but a 152 main during summer conditions (7700 alt dens) should work.

    Charles

    #61839
    Scott Mowrey
    Participant

    Wow, thanks for the feedback. I’m going to try to get out to the track this weekend for some experimenting. By the way, a little off-topic, but why would they not open the track at the crack of dawn during the summer or keep it open later to avoid the mid-day heat?

    #61840
    Greg Welch
    Participant

    We are racers, we can’t get out of bed early!

    We always run the k27 needle around here, the k98 has always been significantly slower for us. We also run it in the 2nd clip down just like my dad said. The problem you described to me sounds like the normalities of a Rotax. With the hairpin at Centennial, its all about getting the low RPM up. In the CW direction it should be around 7500, and around 7200 in the CCW. As long as your above 7 it will pull great.

    #61841
    Anonymous
    Inactive

    Bobby & Brad can’t get up that early to open the track. 😥

    They need there beautifying sleep 😆

    #61842
    Scott Mowrey
    Participant

    I checked the carb last night and it’s a K27 needle on the second position. OK, well, I guess I need to practice some more….the best I can do around that damn hairpin is about 6200-6300 rpms.

    #61843
    Anonymous
    Inactive

    Either brake later into the corner

    or

    brake sooner and get on the gas quicker

    It’s a corner that will test your nerve

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