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Exhaust: How does it help/hurt?

Discussion in 'The Garage' started by JoMoThumper, Jul 17, 2007.

  1. JoMoThumper

    JoMoThumper Los Bombero Caliente

    Joined:
    Apr 15, 2007
    Oddometer:
    260
    Location:
    SW Mizzou (J*Town)
    My bike: '97 KLR 250, 11k miles (although my question is not about this bike in particular).
    My story: Had the carbs cleaned by a friend since the mere thought of opening one up makes me face my own limitations as a man. He told me it was pretty dirty, probably never cleaned before this. Only other change was swapping out the stock exhaust for one the P.O. gave me w/ the bike as an extra. Comparing the two, the one on the bike now is much lighter and looks better as it has no rust. Thus the swap, so I can clean up and re-paint the stock pipe. Bike sounded great, more of a "thump-thump" sound in place of the stock "puff-puff".
    My question: How does the exhaust set up influence engine output? Seems as though the power band on my bike flattened out and lost a bit of "get up & go". I was surprised since the carbs had been cleaned out, I expected a little more kick as it had been stumbling though the 5-7k rpm range. The engine definitely runs smoother, just seems flat as it throttles up.
    As I am a self-proclaimed non-genius I throw this open to general interpretation, gentle heckling, slight sarcasm and any insightful banter.
    #1
  2. ShadyRascal

    ShadyRascal Master of None

    Joined:
    Feb 14, 2007
    Oddometer:
    12,217
    Location:
    Whippin' Scutt Farkus
    You need to address the jetting. When replacing the corked-up stock pipes with free flowing pipes you need to richen up.
    #2
  3. JoMoThumper

    JoMoThumper Los Bombero Caliente

    Joined:
    Apr 15, 2007
    Oddometer:
    260
    Location:
    SW Mizzou (J*Town)
    From what you said I assume that merely increasing airflow can mess up the air/fuel ratio somehow? Resulting in what I have?
    Would backing out the air/fuel mixture screw do anything productive for my problem or is re-jetting the only or best route?
    Since this set up on my bike is temporary while I freshen up my stock exhaust, will running this non-OEM present any possibility of damaging anything?
    #3
  4. ShadyRascal

    ShadyRascal Master of None

    Joined:
    Feb 14, 2007
    Oddometer:
    12,217
    Location:
    Whippin' Scutt Farkus
    The air fuel mixture screw is for idle and just off-idle, so it won't do much overall. If this is temporary, you're probably all right. If you're way lean it'll start running hot, watch for that.

    Letting more air out draws more air in, and larger volumes of air demand more fuel to keep ratios right.
    #4
  5. JoMoThumper

    JoMoThumper Los Bombero Caliente

    Joined:
    Apr 15, 2007
    Oddometer:
    260
    Location:
    SW Mizzou (J*Town)
    So, it's kinda like my dog? The more she eats the more she poops? :D
    Got it, makes sense. I did watch the temp gauge on the way home from the shop, seemed to be about where it was before. Thanks for the tip.
    What if I taped up some of the air intake in the mean time?:wink:
    #5
  6. Laromonster

    Laromonster Lean, mean pacifist machine

    Joined:
    Jun 28, 2004
    Oddometer:
    2,095
    Location:
    Behind you
    just guessing that it may be lean is not such a good idea, pull the spark plug and check the color of the electrode before you start taping up air intakes.

    A free flowing exhaust gives you HP at the top of the rev range, a less flowing system adds torque.. could be an explanation for the lack of snap you experienced
    #6
  7. JoMoThumper

    JoMoThumper Los Bombero Caliente

    Joined:
    Apr 15, 2007
    Oddometer:
    260
    Location:
    SW Mizzou (J*Town)
    I replaced the spark plug a week before the carb work.
    Taping up the air intake was just an attempt at a joke, I'd never do that especially since the stock pipe will go back on in a few days.

    #7