Home Forums 2-Cycle Racing Walbro Wb3a / KT100 tuning

This topic contains 4 replies, has 2 voices, and was last updated by  tony zambos 1 year, 10 months ago.

  • Author
  • #83801

    Andrew Kutscher

    KT100 and karting in general is new to me.  Long time PCA driving instructor and track guy who decided late in life the best way for me to go racing was to get into karts.

    I’ve read quite a few old posts on this topic but I’m still a little hazy on a couple of things.

    Seems that the general rule of thumb is as follows:

    High: 1/8 to 1/4 turn

    Low: 2 – 2.5 turns

    My general understanding is to go out on the high jet until the motor starts to 4 cycle at the end of the longest straight and then lean it out slightly to stop that behavior.

    Then to start to lean out the low jet until the motor pulls less coming out of the turns and then finally to monitor temps to insure the motor isn’t soo lean that it burns itself down.

    I’m a big data guy, I want to get into race stuidio between sessions and look at actual data to confirm maximum performance. Basically break the track into sectors and work on performance out of the slowest turn on the course and then also work on MPH at the end of the straights to confirm these adjustments.

    So is a general statement correct that overall mixture on high speed stuff should be done with the Low jet and torque / performance out of slow corners should be done with the High (flex) jet?  Would I ever go back and adjust the low jet for any condition on corner exit or for overall temp control?

    If I take this approach for maximum performance is it just going to be a waste of time because the motor will get soo hot that I’ll have to richen it up anyhow to keep it healthy?

    **I realize I’m ignoring factors such as clutch engagement but I want to keep this strictly to carb principals**

    Also, I do have a popoff gauge and fulcrum height tool so that I’m sure I have a good healthy setup on the carb for this baseline testing.

    Thanks in advance for any assistance

  • #83808

    tony zambos

    Your basic idea is OK for the starting positions of the jets. You’ll probably get down to an 1/8 on the HI end although leaving it a 1/4 is safe for starters. You may even get down to almost having the HI end closed. It will depend on the length on the straights. The LOW and HI end jets have different profiles. Leave the stock HI end in the carb and replace the LOW end needle valve with the flex-T. You’ll do most of your tuning with the low end. Never crank the needle valves into their seats. Turn them gently into their seats and never force them past that point.
    You didn’t say whether you’re running a pipe or can muffler. Either way, suggest that you install an EGT probe in the exhaust. Don’t bother to trying to read the temps as you’re driving as it will be changing faster than you can keep track off. But after a few sessions, you’ll see in Race Studio what your highest temp is and then set an alarm light on your gauge to slightly above that. That will help avoiding a stuck piston, especial with a pipe. The EGT should rise as long as you accelerate. If it drops, you’re too lean. If it rises in corners, the kart is over stuck.
    As for pop-off, some racers will say it’s important, others say you need some. 9.5 psi is a good place to start. Spray WD-40 on the needle valve when testing the pop-off. Use a paper towel over the open end of the carb so you don’t get soaked with WD-40. The fulcrum height should be .045″ below the carb body without the gasket.
    TIP: Don’t plan on the engine to make you the fastest guy out there. Driving and chassis setup is where the performance is.

    Good luck and have fun

    LAD Specialties customer / tony kart / rotax / kt100

  • #83809

    tony zambos

    Here is a link to a video on checking pop-off. It’s not for a Walbro but the idea is the same. http://kart360.com/features/tech-tips-presented-by-p1-engines

    LAD Specialties customer / tony kart / rotax / kt100

  • #83833

    Andrew Kutscher


    Thanks for the tips I really appreciate them.  I’m working on the driver and setup issues trying to get comfortable in the kart as it’s a little bit of a transition from my GT3 but has been a fun challenge so far.


  • #83852

    tony zambos

    I’m guessing, but a big change for you is the lack of a rear differential. It’s just mere combination of caster, chassis stiffness and axle. :)
    You may have not realized how critical seat placement is. Check you chassis manufacture’s web site for recommendations. If it’s near the recommendations, leave it for now, but keep that in mind if you’re struggling with handling issues.

    LAD Specialties customer / tony kart / rotax / kt100

You must be logged in to reply to this topic.