Jetting to resolve Cold Blooded condition

Discussion in 'WERA Vintage' started by pscook, May 18, 2020.

  1. pscook

    pscook Well-Known Member

    I have a 1990 CB125T (125cc twin used for MSF training) that is the most cold-blooded vehicle I have ever owned. One "feature" about the carbs is that they are the same carbs for this 125 twin and the 250 Rebel, so lots of air for the 125 (or too small for the 250?). Regardless, this thing sucks to ride until it warms up after 10 minutes of riding. And then it's so anemic that

    Fresh gas, cleaned carbs, good plugs, spotless air cleaners (and no rags in the intakes). Do I go richer or leaner on the pilot or main to get it to warm up faster? Or can I get away with just adjusting the mixture screws?

    I'm all about experimenting with jetting (lots of two strokes in my life), but these carbs SUCK to remove. It's like they bolt the carbs to the engine before they install the engine. I almost want to pull the head to make it easier to play with the carbs. So I want to make as few internal adjustments as possible. I'll play with the idle mixture screws first, but I don't know if I should go richer or leaner.

    Thoughts or suggestions?
  2. grasshopper

    grasshopper Well-Known Member

    I'd go fatter on the idle circuit / pilot jet. Does the carb have the mixture screw on the air or fuel side?
  3. VFR#52

    VFR#52 Well-Known Member

    Open up mixture screws to 2.5 turns and go up 1 size on pilot jet and problem will go away.
    Dont mess with needles or mains.
    Best of luck.

    DmanSlam likes this.
  4. DmanSlam

    DmanSlam Well-Known Member

    Agreed that you probably want to go richer (and that this is not a needle or main jet issue).

    Let your mixture screw be your guide and confirmation: If your AIR screw is in less than 1 turn (or your FUEL screw is out more than 3 turns), you need a richer pilot.
    VFR#52 likes this.
  5. pscook

    pscook Well-Known Member

    Mixture screws on the fuel circuit (four stroke carbs). No air screw. I just needed to confirm that going richer is the direction to pursue before I start practicing my swearing and bleeding in preparation for removing the carbs.

    I'll roll the idle mixture out a bit to see how it reacts, at 1.5 currently. Luckily there are ports for vacuum gauges to balance them out, but I don't think that I have the adapters.

    And upon a little reflection, as it's the down tube/non-cradle engine mount style, I could remove a couple of bolts and just rotate the engine down for easier carb access. Not that I WANT to, but if I HAVE to access the carbs to bump the pilots.
    VFR#52 likes this.

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