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Carberator A/F ratio tuning.

Discussion in 'Motorcycle Tech Talk' started by Merel Jones, Jul 5, 2017.

  1. Merel Jones

    Merel Jones Member

    Apr 18, 2016
    Likes Received:
    I am riding a 2002 Harley Dyna with the 40mm CV carburator. About a two ago I added a K&N free flow air filter, RB Racing two into one exhaust and ss gear driven 510g cams. At the time I set up the carberator with CV Performance needle, emulsion tube and jets. I rejetted the carberator to a 46 pilot jet and a 185 main jet. After doing this work there was a noticable drop in MPG. Dropped from 45ish to around 40ish. But the bike has run very well.

    I recently had to do some work with RB racing and bought an A/F ratio gauge kit from them. Now it appears that I am running somewhere around 12.7 throughout the trottle range. I was able to get the idle speed A/F to bounce down into the 13.2 range, but intermitently. I have found that while riding in the morning with temperature below 60F the A/F ration will at times be at the 13.2 point. This is for brief periods but it is noticable. As soon as the air temperature rises A/F again goes to around 12.7. So overall I am in the rich side of things.

    First thing I am curious about is that I am surprised that I am running rich with the jets I have installed. Any comments on this?

    Now I am looking at dropping the pilot jet to a 45 and the main to 180 to see what happens. Anybody hazard a guess about what might happen? Yes I know it should get leaner.

    Some interesting observations:
    1. During decelleration, going against compression, the idicator will show a fluttering in the lean area and at times will go pretty lean for a very short period of time
    2. At mid trottle positions there is a slight change in A/F to the lean side then stabilizes back at the previous point.
    3. There seems to be a sweet spot at about 2500 RPM where the A/F will stablize in the 13.2 range but any trottle deviation will move back to a rich condition
    4. Again, with lower temperature and more dense air the A/F moves to a less rich condition.

    Interesting stuff. Hope to get some feedback. I don't understand all of what I am seeing but I haven't ever seen any of this before.
  2. Red Rider

    Red Rider Well-Known Member

    Jul 5, 2006
    Likes Received:
    Churchill County, NV
    I suspect that most folks have read your post and scratched their head like I have. That said, my feedback is:

    1. More power usually means lower mpg. You increased airflow, hp and torque. That airflow has to match up with fuel flow. That 510 cam was not put there to optimize mpg, either.

    2. No offense intended, but 99.999% of the readers here don't have, need, nor want an A/F ratio gauge kit (JohnnyBiker is a certain exception!). IMHO, what you are observing with it seems to be consistent with your 510 cam and airflow.

    3. Ride that Dyna and tell us how it feels! Wrenching is cool, and respected, but riding is way cooler.
    MountainCruiser likes this.
  3. mpz

    mpz Member

    Aug 29, 2008
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    You've just installed a run rich kit. Bigger jets take more fuel. If your bike is still 88ci, you don't need jets as big as you've got. Plus you added performance cams that make power at a higher rpm than stock, so you have to use more fuel and a lower gear to make your bike move, especially at low speeds. Everything about engine timing is a compromise between fuel economy and performance. You're leaning toward performance in every aspect.
    Red Rider likes this.
  4. hotroadking

    hotroadking Super Moderator Staff Member

    Jun 16, 2004
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    Mouseville USA
    Everyone knows its wrist position that causes increases or decreases in fuel economy, and typically performance parts cause the wrist to rotate further and farther for longer duration, as well as more often than a stock bike....
    MountainCruiser and Red Rider like this.
  5. timsmcm

    timsmcm Member

    Jan 16, 2017
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    100% right my man.
    Red Rider likes this.
  6. charlie46

    charlie46 Well-Known Member

    Mar 4, 2010
    Likes Received:
    Melbourne Fl
    very well said HRK.

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