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  1. #1
    Sugarcubes
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    Default low rpm stutter - pilot jet?

    My bike has a 1994 evo 1340 motor with EV27 Cam, paughco upsweep pipes with baffles and a screaming eagle K&N high flow filter. I am running the stock CV 40mm carb with a 45 pilot jet and a Dynojet ket with the DJ170 (equivalent to 190 keihin) main.

    Been playing about with the carb and needle settings to try and get the best mix of smooth daily riding and strong acceleration.
    If I turn the idle screw out 1.5 turns from seated and run the Dynojet needle on the 2nd clip groove, the bike runs super smooth at low rpm and I can effortlessly cruise on quarter throttle / low rpms in traffic. However when I open up the throttle, whilst there's no sputter or cough etc she doesn't pull all that hard. Plug chop in this condition shows a fairly white porcelain, and dark grey electrode, black around first couple rings of thread as usual. So is obviously on the leaner side.

    I put the clip on the 3rd groove on the needle and it pulls better in the midrange and gives a bit more colour on the porcelain. However riding about town in traffic it develops what I can only describe as a bit of a jerky sputter at low rpm rather than a smooth throttle and it doesn't smooth out unless you give it more throttle.

    Turned the idle screw out 2 turns from seated (any more and it starts to blacken the plugs) and popped the clip on the 4th groove on the needle (Dynojet recommends the 3rd slot but I wanted to see the difference) and the bike hauls ass on the motorway, really feels pretty quick now. However the low end sputtering is worse.

    Reading up on similar issues leads me to believe that the 45 pilot is probably too small and the sputter is the result of the throttle curve transferring from a comparatively small pilot jet to a richer main jet / needle combo. Richening up the idle mix doesn't do much other than increase the fuel load at that part of the cycle as the pilot jet is still too small when under load.

    Does this sound correct? Im going to put a 48 pilot in it this week to see if that smooths things out
    Last edited by Sugarcubes; 07-24-2019 at 4:23 AM.

  2. #2
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    I think you're on the right track.

    Low speed AKA pilot jetting is critical to low and midrange performance and stock jets are lean due to EPA tuning. Before jet kits were common we used to drill low speed jets on many Japanese carbs with good results.

  3. #3
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    Did you drill the bottom of the slide vacuum hole larger as indicated in the Dynojet kit instructions? Also noted in the CV carb modifications here: http://www.nightrider.com/biketech/hd_cv_mods.htm

  4. #4
    Sugarcubes
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    Slide hasn't been drilled out. There seems to be a lot of people who feel the slide drilling isn't really necessary and to leave it alone. I understand its supposed to increase throttle response but I don't think it would really affect this issue given that it goes away when I lean out the needle.

    I think its probably stuttering as it transitions from a 45 pilot to a fat needle and big main jet. No air leaks either (i made sure of it lol) so I am hoping the 48 pilot will fix the issue!

  5. #5
    Sugarcubes
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    Quote Originally Posted by Sugarcubes View Post
    Slide hasn't been drilled out. There seems to be a lot of people who feel the slide drilling isn't really necessary and to leave it alone. I understand its supposed to increase throttle response but I don't think it would really affect this issue given that it goes away when I lean out the needle.

    I think its probably stuttering as it transitions from a 45 pilot to a fat needle and big main jet. No air leaks either (i made sure of it lol) so I am hoping the 48 pilot will fix the issue!
    Got the 48 slow jet in this evening. Fine tuned and the stutter is gone. Nice and smooth throttle again 👍

  6. #6
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    If you find the 48 is too big, CV Performance offers a 46.

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