Richening low RPM while keeping high RPM same

LO206 red slide cadet. Mychron5s with LCU in a test pipe measuring and logging AFRs. After extensive testing and logging I can get and observe changes to AFRs in different weather conditions and dial in almost perfectly with needle and air screw adjustments. I have not changed float heights while logging and found mine to be 0.91" when I got it back from the dyno.

In this particular situation when weather is warming and air density is dropping ~95% I find I can get the 4000-6100 RPM AFRs at a steady 13:1 which seems right from the relative speed to the other top competitors (with professional mechanics supporting them!) but my AFRs at 2500-4500 are 13-13.5:1 and leaner than I understand is the ~12.25 - 12.5:1 target down low.

I’d appreciate some help shortening my experimentation cycles here…whats the proper approach to richening up down low without losing what I think is the right top-end mix? Raise the float height a touch, keep the needle clip where it is and open up a half turn on the air screw?

Does float height influence high or low-rpm more?


Float height affects everything, low and high end.

Fuel mixture screw changes the bottom end and the needle adjustment changes bottom and some mid range.

Set the mix at 1-1.25 turns out, float height around .870, and needle set at 2nd position and let it go!

Thanks Myles, will test along those lines. I found the needle height made massive changes…in Heat 2 he was being outrun on the long straight, popped up a clip for the final and he gained 4 kph on the straight and was keeping up/reeling in competitors. Will try raising the float height, keep the needle in the same spot and adjust with the air screw to see if I can keep that sweet top end but find some more more low end torque.

With all due respect, there is not a carb change you can make to pick up 4 kph in a red slide cadet. Focus on your driver and your chassis tuning and rolling corners. That is where your speed increase came from.

You are chasing thousandths of a second with carb/engine tuning. There are tenths and whole seconds in driving and tuning a cadet kart.

And I am saying that as a 206 engine builder that would love to sell you my latest carb tune or my latest tuned engine. But its just plain not worth it.

Myles carb tuning/understanding is very good. I would set the carb to his recommendation and you will be about as close as you can get.


Its worth noting that you need you know what your fuel pump output is like, fuel pump pressure is very different from pump to pump and between the 2 part numbers it is different. You fill the pump vent and it also changes pump pressure and as a result the tune.

All pumps are under 3psi but even a small change has a big effect on performance.

This was my first setup before getting a 3psi max res gauge.

Pump pressure is pretty similar to changing float height in terms of what it does.

I’m with @fatboy1dh on this one. I have never changed my carb settings unless I develop a stutter/miss then I’m resetting float height back to where it’s supposed to be because it probably got jostled enough to change.

Saturday, I went from Derek pulling me out of 1 and all the way down the straight to running dead even with him just by getting thru turn one better with a minor chassis adjustment. The speed pickup was probably 2mph+ on corner exit which means an extra 2mph+ over the previous seession the entire straight. I’ll dig out the mychron and confirm later.

Odds are the track came to the kart or your driver drove better.

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What changes and effects are you observing?

I’m having a tough time understanding how pressure that’s going to a vented chamber has a discernable impact. I guess it could affect how the needle closes, but there’s a LOT of leverage on it from the floats.

4KPH is HUGE. I think if you overlay the speed traces you’ll find some clues on what it came from. It does sound like chassi/driver/draft (pick any combination) vs the needle change.

To continue the AFR rabbithole… have you been tracking air temp and density?

Thank you for all your interest and replies.

On the data front, yes I am logging GPS speed, XYZ accelerometer, RPM, AFR and a bunch of other channels. Throttle position is just about the only one I wish I had but dont.I am a rookie karting Dad but a veteran aircooled 911 tuner.

I do track air density, temp, humidity, needle clip, screw turns, AFR ranges, float height, wind and track temp for each run.

I philosophically disagree with the argument of there being bigger gains elsewhere so leave it alone for 3 reasons.

  • #1 We missed the podium 2 weeks ago by 0.007. So yes, I am chasing thousanths in a field of 22+ karts against some professional mechanics.
  • #2 Rest assured I am chasing those other driver development and chassis development.
  • #3 - I am always going to give him the best ride possible

I checked the data again from Saturday and it is indeed a difference of a81.4KPH and a84.2KPH max into turn 1 at Goodwood which is the fastest corner on a 900m straight otherwise. So a3KPH but up to 4.1 KPH on turn 4.

His lap times are very consistent. Traffic in a 22+ field is always a factor, but he puts in 10-15 laps at a time within 0.3 seconds depending on where his is experimenting on faster lines.

This track has half its length in a straight with a kink and the other half in tight corners making tuning particularly helpful. Nail the low speed corners and launch, nail the 1200m at full throttle above 4000 rpm and I am happy.

As a point of interest which have not tested yet but am keen to hear from those who have…why not start with maximizing air intake (slide at max legal limit), air screw all the way out) then then use float height and needle clips to tune into range for the course idiosyncraticies and weather conditions?

Maytbe that just ends up where you generous repliants have suggested.

I dont think anyone is doubting your data. That is absolutely possible and props to you and your driver for making it happen. I think we are questioning your causation.

What was his corner exit speed from the previous corner? All I can say is having these long slide engines (red, green, purple) on the dyno, there is not enough hp difference in carb tunes on a red slide to make up that much more speed. You could go from clip 1 to clip 5 and not see that much difference.

Yes, this is a great way to tune. These classes, by definition, are short on air. You need to get maximum air in the intake to make them go.

Its hard to see these minute changes on the track. Find someone local with a dyno that can help/let you get the engine dialed in. Then you will have one less thing to worry about on race day.

I’d be curious to see the speed and RPM traces of the two fastest laps overlayed.

It’s been known for people to play with the idle air screw alright.

Thanks Derek. That is very helpful thanks. I did have the local dyno guy do some work which found some power but that was on a cold wet day. Also he broke a stud in my head, and stripped the exhaust header threads in another cart, and he has allegiances to other racers too so I feel I need to own this myself…

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Hi Jim, I will post some more traces when I am on the laptop thanks.

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Hi Jim,
Here you go, thanks for the consideration and thoughtful advice.


I just want to note that while the two compared lap times are similar in overall time, there is a fair bit of variance in braking points, apex speed, and especially minimum apex speed in that final corner.

While I can sympathize chasing the last hundredth, the reality is most kinds in this age range, even the really good, national-level ones, are making mistakes almost every lap. There is lots of room for improvement in almost every kid this age, and even in most pro-level adult drivers.

You do seem like a data loving guy though, so if this is fun, rewarding, low-hanging fruit for you by all means let’s figure it out. But I just tend to agree with Derek that there is far more to find in driving than in this specific tuning aspect. If you lost out by 0.007, I think there’s a fairly reasonable assumption to be made that you could find more than 0.007 in driving improvements over the course of 20 laps.

Anyway just wanted to say my piece, carry on.



99 times out of 100, when I’m at the track and contemplating doing any changes or blaming an engine thing, I normally end up falling back on the quote from the good ol’ GOAT#10:

“Drive Better.”

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Thats just a fact, I want to be angry about.

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Thank for that.

There’s a big variance on the minimum/apex speeds, but the power seems to be very close. Entry speeds are up at times also, but more due to the driver going deeper

I think you might have to go one lap back on that higher top speed to see if it was the result of a faster exit from the final turn.

It’s hard to attribute anything to a power difference looking at the graph because of the speed variances in the turns.

That doesn’t mean you can’t continue to pursue getting the most you can from the engine of course.

Might be worth reviewing any GoPro footage if you have any to see if the variances were driver errors, or just dealing with traffic

@tjkoyen goodwood is very similar to Badger classic. Designed out of the same IKF track book. We know based on Badger, “no mans” makes or breaks top speed into turn 1.

As you mentioned the trace shows more as to why the top speed differences with corner speed.

Definitely love having a data guy digging so deep into a 206 though. I’m all ears for this to continue. I plan on learning something. :+1:

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