3500 min RPM with yellow springs?

I need some guidance here…LO206 red slide B&S Cadet. The standard around here is a Flame clutch run sprocket outboard on Intrepid Cubs, Tonys and Birelarts. Weight is 230 lbs.

Our Mychron 4 shows max rpms which I use the gearing to dial in for the particular track, weather, tires, etc usually around 6050. I have my min RPMs on hot laps are ~3500-3900. I was told to tune the clutch to get the minimum RPMs into the peak torque curve which on a red slide is ~2800.

I was also told to go from black springs to black/white and then to all whites. Today I went to white/yellow and finally to all yellows and still my min RPMs are ~3500-3900. Leading feet.

I also have a new Fire clutch so have switched those springs back and forth with the Flame and find no difference. Both the Fire and the Flame with yellow springs show a min RPM in the high 3000’s, even though the Hilliard site has the engagement speed at 2200 (noted this is the engagement not the lockup speed.)

Am I off track, or is there something else going on? Should I be trying orange springs, or weights in the clutch feet? Am I right to be targeting a much lower min RPM as a gauge for where the clutch engages off starts? My son consistently has slower starts off the rolling start, but I suspect a mechanical shortcoming in this situation here as he is a consistent high finisher by the last lap.

Hilliard Clutch Engagement Speed Chart

Can you help guide me towards the right cluch engagement strategy and if my 3500 - 3900 really needs to get down to a 2800 min RPM?


Silly question. Are you sure your clutch is disengaging on track? It may be that the slowest corner is not slow enough to disengage clutch. If that’s the case, springs won’t matter. Your lowest rpms will equal your lowest engaged engine rpms.


I hadn’t thought of that when I first read this. @JohnJL what track are you racing at? That will play a part in the clutch engagement. I will also say I know 206 Cadet you only want to be hitting 5800 RPM or so, so you may be geared too high to be disengaging the clutch.

Hi Guys, thanks for the responses.

I am not sure the clutch is disengaging, I consistently get a 35-3900 min RPM at the end of the session on my Mychron. I would expect to see it dip when he hits the gas off the slow rolling formation lap…with all yellow springs it seemed to bog down and he got better starts with white/yellow springs. If it wasnt disengaging should I still expect to see a low RPM off the start even if it stayed engaged the rest of the session? As I typed this it just occurred to me maybe the Mychron is throwing out the rolling start from the 1st lap because the “go” happens before the start/finish line?

We were at 3S Karts this weekend but got the same at Goodwood and Shannonville…Interesting you say the 5800 figure…the mechanic who is supposed to give us good advice (he is employed by the “Team” but also personally looks after another kid in my kid’s class) says to keep tuning for the rev limiter, which I do through gearing and leaning it out if its hot. Another Dad with a fast kid and kart said at the end of the weekend he was also geared way lower that we were. He was like 3.4-3.4 where we were 3.6. They end up being around the same pace…My kid is at the pointy end of the field, fastest rookie and between 3-6 place in a field of 20 depending on who shows up…but those #1 and #2 kids clearly have something else going on and guess who looks after those karts :wink:

Thanks for taking the time to read and respond! I need some advice from someone who isnt competing directly against my kid :wink:

Don’t chip the red slide.

You’re looking at tuning the clutch/min rpm wrong. Put the springs in for an RPM slightly below the peak torque so it’s full lock at peak torque. You’re not changing springs to bring the min RPM of the engine down.

You mychron is gonna show you the min and max RPM for the lap. You gotta go to the full run view and scroll all the way to where you fires the engine and go through the graph from there if you wanna watch the initial clutch grab.

Hi Matthew, thanks for the reply. I may have not done a great job explaining myself; I agree with your approach. My revelation has been that perhaps my clutch is never disengaging during the mychron laps. I found a few “out lap” logs that I have identified as his formation and start lap logs. They showed his RPMs dipping down to ~2000 off the start with all yellow springs. For this weekend I put back in 2 yellow/2white springs in the clutch.

I also suspect I have been gearing him too high. Under guidance I described above I have been gearing him to hit the rev limiter at 2/3rds down the main straight. The tracks he is on (Goodwood, Shannonville, 3-S, Point Pelee) are 2 long straights but most of the tracks are tighter and technical for at least half their length. I am going to try dropping a tooth or 2 and aim for max RPMs on the straights of 5,900?

Thanks again for the time and advice!

Also, what does “Dont chip the red slide” mean?

It means on most tracks, the fastest way will be to not let a red slide engine get to the rev limiter. If you are getting to the rev limiter, you need to remove teeth from the rear sprocket.

Thanks. Thats exactly opposite of what the Team Coach and guy I paid hundreds of dollars in coaching told me, but then again his son is competing against mine and not winning by much…maybe surprise him this weekend :wink:

To try to teach a little better, the long slide engines (purple, blue, red, green) do not have much power when you get to higher RPMs. They just cannot pump enough air at those RPMs to make any torque. Because of this, you will often find that keeping them lower in the RPM range results in lower lap times.

Now, like everything in racing, nothing is set in stone. There are tracks and situations where you will want the acceleration of a larger rear gear or smaller front gear in order to gain time through the slow portion of the track. The best i can tell you is try them both and see what you like.

For visual representation of why we keep long slide engines lower in the rpm range, check out their torque curves and hp curves compared to a senior engine (black slide):


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So vertical axis is torque and horizontal axis is RPM?

The blue curve is what relative to the red curve? How does one understand a dyno graph?

Yes, torque/hp on the vertical axis. RPM on the horizontal. Red line is torque. Blue line is HP. As you can see, comparatively, the slope of a red slide torque line goes down quickly compared to the line of a senior slide.

Thank you. I was thinking blue was “before” and red after.

So the blue line is senior slide or no slide? Also, presumably the “slide” is exhaust related, like on an x30 junior?

Red line is engine Torque with the red slide, blue line is HP with the red slide. On the full sheet they have one of these graphs for each carb slide they offer.

The slide is in the carb. 206 uses a slide carb instead of a butterfly carb.

Tried a different gearing strategy this weekend and it didnt go well. On a track where he usually logs his fastest times and keeps up with P3-4 with a peak RPM of ~6050 (he says it hits the limiter) and a rear gear of 65-66. THe track had a temporary chicane in the middle of the straight this time so I usually up the teeth in back to ~68-69, and again RPMs end up ~5900. I tried leaving the gear in back at 66 to get it in the published HP and Torque curves and his peak RPMs when he came in were ~5300. It mechanically did what I wanted and expected but his qualifying was terible…14th place where he is usually 4-6th. Through H1 and H2 and Prefinal I kept upping the teeth to bring RPMs back up near redline and his results got better and better. Unfortunately he got pushed into the wall, which bounced him to the back of H2 and then he dropped his pushback nose in Formation lap when the back accordioned into him, for an automatic 3-place penalty. That dropped him to 18th place start for the final but between the higher gearing (5900 RPMs) and some pretty solid driving he got it back to 10th. Anyway, back to the original observation…Mychron still shows min RPMs of ~3300!

Post a pic of your RPM lap graph.