Off idle flatspot

Discussion in 'Engine, Fuel and Exhaust' started by Alaskarider, May 13, 2011.

  1. Alaskarider

    Alaskarider New Member

    My first post and I am pleading for help! First of all, thanks to all the people who contribute to this forum because it's great and a wealth of information.

    My wife's 2005 883 Sportster has the SE air cleaner and slip on pipes w/out baffles. It has 2200 miles on it and according the p/o it was jetted for the pipes and air cleaner. The carb is stock other than the Dynojet kit.

    We finally got it on the road (Alaska weather and gravel!) and it runs just OK with the enricher halfway out after warming up. With the enricher off, there is a huge flatspot just off idle and I can't get it past 1/4 throttle. If I quickly twist the throttle several times (using the accelerator pump) and get it past the flatspot it runs strong.

    I opened up the carb and it has a #42 slow jet and a #180 main. (Dynojet) The needle clip is centered and the inside of the carb was really clean and I verified that all the passages were clear. The float is set correctly also. I checked the diaphragm and no cuts or holes. The slide has NOT been drilled. The accelerator pump is working and the slide moves up smoothly when I give it throttle while running.

    Is the #42 slow jet too small? Any suggestions before I start throwing $$ at it?

    Sorry for the long winded post for one question.


    BUBBIE Well-Known Member

    The MORE information we have the better we can answer...

    Without looking and riding your 883,,,, I say the 42 is too small and I would go to about to a .044 - .045 might be too big but the 44 min. for the idle and I don't like NOT having Baffles in the exhaust... This by its self May be the problem with a flat spot...

    Running really lean in the idle/slow end MIGHT be a vacuum leak at the intake also....That is a lot of enricher to get the gas into that little motor...

    If the 44 45 don't do the job it could be a clog in the slow jet area and need to check that...DON't drill the slide(not drilled is good)

    The needle can also be shimmed up by using One or Two #4 copper washers to raise the needle.... to get the fuel there sooner when twisting throttle..

    I really think IF it runs smooth on the high side and Only has a flat spot ,,,,,the increase in the slow jet and the raise in the needle may be all that is needed to try First ...

    Not suggesting to spend $$$ just try the little things first.. I personally Never have liked the dynojet.. I use Stock HD carb and have had better results that way..

    I have a carbed 06 1200 sport R with python 3 exhaust and Big Sucker air that runs great. I use a .046 /.048 jet (Wa./ Az.)so I'd be surprised if you need bigger than a .044 slow speed jet in the 883..

    Give what i suggested a toss and see IF you get results..

    Where does the idle screw end up in turns out to idle?

    It should be at about 2.5 turns open when using the correct slow jet... Having to open more or close more than the 2.5 means the jet is either too big or too small..

    easy enough to check.. Get back and more help for you later..

  3. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    If possible i would get rid of the dynojet kit and return to the stock jets and slide needle
    As Bubbie said 42 for the slow jet is perhaps too small and a 44 or 45 much better and perhaps a 185 main jet
    However i live in northern Scotland and under certain climatic conditions found that i had flat spots backfiring and engine cutting out using the stock cv carb eventually tracked the cause to carb icing when there was a high moisture content in the air
    The moisture in the air freezes in the carb and blocks or partially blocks the jets this would occur in temps between 3 and 15 c
    To remedy my problem i replaced the stock cv with a mikuni hsr on my 2 big twins however my sportster seems to be ok with a re jetted stock cv

  4. Bodeen

    Bodeen Well-Known Member Staff Member Moderator Contributor

    Welcome to THE forum. These guy's above have you covered with good advice. I second the idea of ridding yourself of that Dyno thingy. I suffered fro the same thing when I purchased my bike. Mine was drilled. I got it all back stock and after some patience, trial, and error I got it running good. Hang in there and it will all get sorted.

  5. Alaskarider

    Alaskarider New Member

    Thanks for all the great advice. The more I read about the Dynojet kits the more I don't like them.

    I didn't think about the lack of baffles but that makes a lot of sense. I am going to look into getting a set for the pipes.

    I acquired a 45 jet today after 4 places! I am going to give it a try and I will post with what I find. I couldn't find a 44 anywhere except online.

    My idle screw has to be around 3 to 3 1/2 turns for the idle to sound normal.

    Thanks again for all the help!

    BUBBIE Well-Known Member


    I had another answer for you but it was WRONG:::

    I was going to tell you your low speed jet probably got a Mosquito in it But that would be wrong Cause a Mosquito can't even fit into the Gas tank let alone the Carb... You Alaskan Members.........:D

  7. glider

    glider Veteran Member

    A 46 low jet would be a better choice. Running the slip ons without baffles will cause problems in drivability as well. You can also shim the needle in the slide up with a washer or to so the fuel curve comes in a bit earlier if you still have a problem.

    That's proof that the low jet is too small. It should end up about 2 1/2 turns out with the proper low jet.
  8. Jack Klarich

    Jack Klarich Guest

    Tricky little screws turned out that far can and will vibrate out, I have seen it happen:s

    NEWHD74FAN Experienced Member Retired Moderators

    Most of the questions have been answered already, I can tell you that with the stock needle, #45 or #46 jet and 2 #4 washers under the needle will do the deed perfectly, only had to back out the mixture screw 1 3/4 turns to get my 2004 Sporty running just fine. :coffee

    BTW, the "bad" thing with the Dynojet kit is the drilling of the emulsion tube...which will cause unpredicatable results in the final A/F mapping. Glider has an excellent write up in the Self Help section on CV Carbs, tuning, et al...! :small3d028:
  10. glider

    glider Veteran Member

    The slide is the part that you don't want to drill because it can give you a flat spot in colder temps.

    *Twin Cam CV Carb Rejet Suggestions - Harley Davidson Community