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Thread: Lack of full throttle response

  1. #1
    jetrep Guest

    Default Lack of full throttle response

    I need some help please. The bike - 2001 XL1200S, 8500 miles, Screaming Eagle cams, two plug heads, K&N oval air cleaner all factory installed. H-D shorty duals removed and I have installed a Kerker (SuperTrapp) 2 into 1 exhaust system. Original jetting was a 45 pilot and a 195 main jet. I installed a CVP stage 1 kit; 45 pilot and 180 main. Mid range performance drove the installation of a 46 pilot jet. Bike is now perfect at idle, off idle response is good and the bike runs fine up to 5/8 throttle, 50+ mph and 3500 rpm in 3rd or 4th gear. From 5/8 to full throttle, the engine "motor boats" sounding as if it is on an rpm limiter, and will not accelerate; rolling off the throttle causes a major back fire. I have had main jets in it from 175 to 195 with little or no discernable performance difference. High speed plug chops show cocoa color plugs.

    I am confident in the mechanical integrity of the carb and correct installaton of the various jets.

    Any advice would be most appreciated. Thank you.

    Paul Juedes

  2. #2
    Join Date
    Aug 2008
    Location
    Blackfoot, ID
    Posts
    121

    Default Something doesn't add up

    What about your jet needle? How was that set? 5/8 throttle would have more to do with that than either the pilot or main jets.

  3. #3
    Join Date
    Sep 2010
    Posts
    7

    Default Yes, needle

    The needle thickness and taper is what controls midrange.

    Think of your finger going in and out of a doughnut hole...


    Your finger is the needle, the doughnut hole is the main jet.

    The needle will be fully clear of the hole at WOT- IF the air flow is sufficient to push the slide up all the way, until then the needle is riding the jet, metering the fuel.

    2. Could also be torn slide diaphram- keeping slide from properly reacting to pressure changes, or wrong spring

    Great CV carb tuning info at nightrider.com


    FYI my 80" evo bagger runs 42/165 N65C needle, drilled slide .125", chamfered leading edge, stock spring----- with excellent performance and mileage. I tuned it using an O2 sensor and voltmeter.

    Mike
    Last edited by mkguitar; 09-28-2010 at 07:23 PM.

  4. #4
    happyman Guest

    Default

    Quote Originally Posted by mkguitar View Post
    The needle thickness and taper is what controls midrange.

    Think of your finger going in and out of a doughnut hole...


    Your finger is the needle, the doughnut hole is the main jet.

    The needle will be fully clear of the hole at WOT- IF the air flow is sufficient to push the slide up all the way, until then the needle is riding the jet, metering the fuel.

    2. Could also be torn slide diaphram- keeping slide from properly reacting to pressure changes, or wrong spring

    Great CV carb tuning info at nightrider.com


    FYI my 80" evo bagger runs 42/165 N65C needle, drilled slide .125", chamfered leading edge, stock spring----- with excellent performance and mileage. I tuned it using an O2 sensor and voltmeter.

    Mike
    have to think a 85 main and a 45 pilot is better for what you have. at least the main . see if it helps then you know if the main is too fat.

  5. #5
    Join Date
    Sep 2010
    Posts
    7

    Default

    Hey, are you sure your timing is retarding when you crack the throttle?

    Does that bike use a VOES to retard timing at periods of low vacuum/advance at high vacuum.

    Mike

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