Lean vs Rich: I try to achieve basic understanding

Oops my bad, I read it too quickly, thanks for clarifying it

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Same guy from above this time explaining the theory of the H and L screw. There are some differences on karts but the theory is the same. Many use an EGT sensor or Lambda sensor to monitor exhaust temps, this also gives more data besides going by ear or feel.

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Unlike in the above video, I was always told by people more experienced than me, to tune the low side needle first. On a carb with two needles, it the one closest to the engine. You do this on the kart stand first, before hitting the track.

Start by setting the needle to a safe, usually manufacturer supplied setting (for both needles respectively) then start the engine, warm it up a bit and start cracking the throttle. Take note of how the engine sounds. Then try turning the needle one direction in 5 minute increments of a clock face. If the engine revs up quicker, you are going in the right direction. If it revs up slower, try the other direction. You continue your changes until there is no improvement in engine response then turn the needle back 2-3 minutes the other way. This errors on the side of slightly rich.

While on track you can tune the high speed needle. On the longest straight, note what your peak RPM’s are at the last point before braking. Are they higher or lower than what you would expect for your gearing? Two, how did the engine sound as it revved? Was it crisp or was it boggy? Did it sound raspy near the top end? These are general clues on which way to turn the needle. Crisp is in the ball park. You can lean the mixture slightly by 2 or 3 minutes at a time and note if your peak RPM improves or worsens. If it improves, keep going in the same increments until no further improvement, then back it off slightly. If it worsens, you are on the lean side and should start to richen the mixture by the same instruments until no further improvement then stop. If the engine just does not want to pull and is boggy, you are likely too rich, but it can sometimes do this if too lean also. Always start with going slightly richer first and if no improvement, you can begin to go lean. Adjust similarly as above until it peaks out, then back it off slightly. If at near the top end, the engine note suddenly changes and sounds like it is double banging, aka four-stroking, you are too rich and the combustion is continuing to happen in the exhaust pipe after the partially spent charge leaves the combustion chamber. Follow the above steps to lean it out accordingly.

James and Aaron make great points about Power vs Mixture not always being straight forward. Sometimes you have to gear for more rear teeth to get out of certain corners and a slightly leaner high speed needle can compensate with the higher rev capability. Other times you want fewer rear teeth and you have to richen the mix to make the most out of the mid range. Like everything in racing, you have to make decisions based on what the stopwatch is telling you.


Perfect. That’s very helpful and a very good explanation.

The basic difference between lean and rich bog is that lean occurs when there is an excessive amount of air compared to fuel. The engine will struggle to accelerate due to a lack of fuel. While rich occurs when there is a higher percentage of air than fuel, the throttle will respond less quickly and with more resistance.

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Welcome and thank you. I think you might have a typo in the 2nd half where you mean fuel vs air.