Cv or Mikuni? / Plus a strange issue...

Discussion in 'Engine, Fuel and Exhaust' started by Doug S, May 23, 2011.

  1. Doug S

    Doug S Member


    94 FXSTC, crane cam, Cyclehshack M series pipes and Revera hi-flow AC..

    Here is what I have: The rear cylinder runs rough, at idle it will fire, fire and then kind of pop or poof, you can feel it by holding your hand over the exhaust. It kind off breaks up or burbles at low speed constant throttle settings, neither decel or accel, just coasting. Other than that it runs like a raped ape, pulls strong through all ranges, idles ok-(other than the rear noise), 40mpg, good looking plugs and no smoke. Compression, leakdown, timing, voes, intake gaskets, exhaust, all ignition parts, either checked out OK or have been replaced.

    Unless anyone has and ideas I'm down to either a carb issue or the cam that the PO installed is either too radical-(overlap) or off a tooth-??. The CV carb I'm dealing with has a dynojet kit and thunderjet, it's been really worked over and somehow it seems to perform well with 45/165 jets, needle clip two from the bottom. The lowspeed needle will not shut the motor off when closed but does kill it when turned three out and the idle speed screw is almost turned all the way in, both of these are incorrect.

    Since this carb has a few extra holes drilled in iit and a ton of work done to it, I'm gonna start over. Question is, should I get another CV or go with a Mikuni 42 and why?

    Thanks for listening
  2. glider

    glider Veteran Member

    Not clear on what it is you are saying here, I assume you are referring to the idle speed adjustment screw?? The CV carb is hard to beat and easy to set up.

    Have you checked the problem cylinder for an intake leak yet? How about the plug wire and plug for that cylinder? I know you mentioned in your post but sometimes things can be overlooked. It sounds like the idle mixture screw isn't functioning properly from your post and could be loading things up a bit at low speeds.Have you checked the plug tips to see if the end is tight where the wire fastens on.
    It sounds like you have a bit too much fuel if you can stall the engine with the idle mixture screw.
  3. ckhismine

    ckhismine Active Member

    Glider is right its hard to beat a CV when it is modded correctly. I doubt the cam is off by the was you say it runs like a raped ape.
  4. karlsbike

    karlsbike Active Member

    I agree with the others - the CV is a fine carb.
    When I did a complete re-build this winter (built a new S&S 96" engine plus other stuff), I also replaced my trusty old CV. Not so much because I did not believe it would perform, but because it was getting worn (the CV piston guides were worn).
    I chose a HSR42 and it seems to be one piece of fine engineering, however as I am still running the engine in I have not explored it fully yet.
    So far I have only adjusted idle and air mix, but it just seems right - very responsive and a good 'feel' to it.
    OTOH, given my new engine, I would probably claim the same with an old Benidix...
  5. Doug S

    Doug S Member

    Thanks for the replies,


    What I mean is that the idle mixture screw normally will stop an engine at idle if it's gently closed all the way, or atleast cause it to stumble badly. Mine does not and I've been running the engine with it just cracked from fully closed. If I open it 3 turns the motor dies, probably over rich. The idle speed adjusting screw is turned in almost all the way and it's probably holding the butterfly open enough that the other circuits are adding fuel at idle and that is why the mixture adjustment does not kill the engine when closed. I thought it might be the ThunderJet but I blocked off the extra fuel line to it and there was no change either at idle or when running. I'm pretty familiar with these carbs and have had it apart for a real good cleaning, tried different jets and needle settings, nothing changed the rear cylinder issue but where it is now is the best for performance. I know the carb is not right and since the PO drilled the slide and also two holes for the Thunderjet, I figure I'd just start over with something new.

    The bike has been doing this since I got it 3 seasons ago, I thought is was just the open drag pipes that were on it but they've been changed. Bike has 31K, compression is 150/ea. and leakdown is less than 5%, no smoke, no oil usage, no leaks, I run Autolite coppers and have tried different sets, intake rubber was changed last year and I rechecked it yesterday with propane, VOES works and does not leak, Wires and coil are new, tried SE ignition, Cleaned out the carbon with water mist and seafoam, I'm kind of running out of easy things to try...LOL

    The real funny thing is that other than idle and lowspeed/neutral throttle settings, the thing really runs good. It's no rocket off the line but it's an 80" EVO, it does not hesitate or bogg down anywhere, it seems to have a never ending top end and it get there with little effort-I never have to go to WOT. I've had it over 100 and had plenty of throttle left, just was running out of nerve..


    BUBBIE Well-Known Member

    My Thinking:

    Put the IDLE speed screw OUT, back where it just Starts to OPEN the butterfly... I'd try a Larger idle jet, .045,46 and get it to IDLE where it should be(position on butterfly)...

    I think "Your Thinking" of being OPEN so far at the butterfly,,, IT IS pulling in Raw gas AROUND the idle system..

    IT should Run set at 2.5 turns out on the "mixture correction needle" IF properly Jetted and Butterfly closed to Proper Idle position...

    TRY and give a shout back..

    I like the 42 mikuni BUT liked my stock Carb. that was set up properly..

    The squirter pump works so good on the mikuni and is easy to adjust to give that quick Pump when needed to increase speed of the motor..
    That was the MAIn difference from my Carb. that I noticed..

  7. glider

    glider Veteran Member

    Looks like you covered most of the bases. Do you know what cam is in the engine? could be an overlap problem at low speeds.
  8. JBC2565

    JBC2565 Junior Member

    Here is a comparision of the 2 carbs. Tons of info on both these carbs in the self help section under fuel and carb tab. Good luck getting her to run better.

    CV vs Mic - Harley Davidson Community

    I've got a Mikuni 42 on my bike and love it. Came with the bike and runs great so never thought about changing it. JMOA
    Last edited: May 23, 2011
  9. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    I fitted an hsr 42 to my softail to try and reduce the amount of carb icing i was experiencing during some weather conditions and it worked well for me also feels to be a better response than on the stock cv i also get fairly good mileage out of it
    Wife was chuntering about her dyna always coughing spluttering and backfiring when it got a wee bit wet so i had to buy one for her also seemed to sharpen her bike up also
    so if i get another big twin it will get an hsr 42 as they do work well

  10. glider

    glider Veteran Member

    Brian is right about the Mik. It will sharpen up the throttle response but when you weigh out the cost difference when upgrading to the Mik, I find it hard to justify unless you had a problem like Brian did with the CV.