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Evo, full cvp kit, low mpg, coughing

Discussion in 'Motorcycle Tech Talk' started by 98FLHP, Sep 24, 2017.

  1. 98FLHP

    98FLHP Member

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    I put a deluxe? Tuner kit on my stock carb and it runs great with minor issues, cough, soot, low mpg when running it harder than normal.
    96 RK evo, stock engine and carb, comp 140 cold both cyls, new intake seals, big sucker, true duals, Bub slip ons, fresh plugs, wires, vacuum lines, fuel lines, voes tested and trips at around 6.5 vac. 185/46 jets, stock slide not drilled, ez just about 3 turns out, idle around 1000 by ear. No black exhaust smoke visible, no strong fuel smell.

    Got about 1000 miles since the rebuild and averaging 35/40 mpg when running 60/65 mph. Got in a hurry a couple weeks ago and ran it 80/85 mph for couple hundred miles. Only got 25 mpg that one day. Normal?

    Get a carb cough at random times, usually off idle when blipping for down shift or when coming up off idle to take off.

    Inside of pipe tips are chrome and soot up to black within a few hundred miles of cleaning.
    Normal?
  2. kenfuzed

    kenfuzed Administrator Staff Member

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    There could be several things at play here, some of which may not be related to the carb or the kit.

    First, the mileage may not be that far off if getting on it, but you could try to drop the main jet to #180 for some better mileage. This may also reduce the carbon accumulating in the pipes. Cleaning the passages between the pilot jet, mixture screw, and front intake hole could help eliminate the random hesitation if there is a buildup inside there. Check out our troubleshooting page for a checklist of common items encountered before and after a carb rebuild.
    https://cv-performance.com/harley-cv-carburetor-tuning-issues

    You didn't mention what the plugs looked like after those few hundred miles (wet, soot/carbon, tan, ashy). This could be a better indication of conditions compared to the pipes.

    Its worth noting that True Dual pipes are notorious for low end problems such as off idle cough, deceleration crackle, and loss of low end toque that leads to the need to accelerate harder to overcome those losses (thus drops in MPG).

    Since there could be other issues not related to the carburetor I am going to move this over to the tech forum for others to chime in on.
  3. 98FLHP

    98FLHP Member

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    [​IMG]
    Plugs don't look bad. Some ash deposit on the left one (rear cyl).

    Carb was thoroughly cleaned when installing the carb kit. All passages blown out, check ball rattling.
  4. hotroadking

    hotroadking Super Moderator Staff Member

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    Number one reason for decreased MPG with performance upgrades is
    wrist position, generally you find your wrist is twisted farther, faster for
    longer periods of time after you improve the performance...

    Plugs look fine, your possibilities are, vacuum leak, carb mounting leak
    manifold seal leak, pipes, cams or a combo of the two, If it was running
    fine before with the same pipes then adjust the idle mix screw, be sure it's
    about 2.5 turns give or take a 1/4 turn from bottom out,
  5. kenfuzed

    kenfuzed Administrator Staff Member

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    I agree these plugs look pretty good. They certainly don't match what I would expect to see if the inner pipe ends were becoming coated in carbon.

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