Simple carb tuning question

Discussion in 'General Tuning' started by Buzz, May 10, 2018.

  1. Buzz

    Buzz Active Member

    Messages:
    446
    I got a 125cc project bike which ran but had issues. It wouldn't idle steadily and struggled to start but once running and warm it was okay. I put an open air filter on and increased the main jet and it now starts well, Idles cleanly but dies as soon as I try to accelerate. Interestingly when I first fired it up it revved very high because I had over compensated from when it idled badly before the new filter. When it revved high immediately I managed to ride it perfectly well. I dropped the idle on the carb to a normal rate and now have this issue.
    If idles fine then idle jet must be good. I Increased the main jet but this shouldn't effect initial throttle from zero to quarter so I can only think that the open air filter is causing this low end bogging but what do I adjust? Is this the slide needle or something else. Is the carb now lean at zero to quarter throttle?
     
  2. Stevep

    Stevep Well-Known Member

    Messages:
    5,895
    "struggled to start but once running and warm it was okay" would indicate that the choke isn't working properly.
    "open air filter on and increased the main jet and it now starts well, Idles cleanly but dies as soon as I try to accelerate" would either be too much air or too much fuel.
    Try putting the standard airbox back on and the standard main jet, put the slide needle on the mid point slot, install a new choke and sort the running from there.
    I have known carbs to be worn out on the inside and no amount of jetting or fettling would sort them out.
    Once you get it right on the standard stuff, then install the open filter, up-jet accordingly
     
  3. Buzz

    Buzz Active Member

    Messages:
    446
    I hadn’t considered the choke, good suggestion. Unfortunately I don’t know the base jetting for this carb, it had 170 stamped on it so not the usual 56-88 range and I don’t know these 5mm jets. It was running but took a while to start so I feared it may be the valve lash however I took a 260 main jet out of a donor carb and put the open filter on and it roared to life with no struggle. It just dies on initial throttle. The fact that it ran beautifully when I had the tickover too high it must have bypassed idle and jumped straight to main jet and ride well. I dropped the idle down to a sociable level and it idles but dies on throttle, I’m thinking too lean on the primary circuit which suggests the needle jet or needle height maybe?
    I’ll do what you suggested first and check the choke although it’s manual and goes in and out properly.
     

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