Telemetry (EGT) and carburetion

Hi everyone,
I’m racing with a TM KZ10b. This is the telemetric data of the last corner (n.12, a fast corner in 4° gear) and of the long straight (n.13).

I see that the exhaust temp goes high and then it become flat and slitghly goes down. The rpm goes at max (this engine doesn’t go over 14.000-14.500 depending on the preparation) and then it become flat for the last metres while the speed curve seems good.

I’m wondering why the exhaust doesn’t reach the optimal window temp (about 630-650) at the maximum. The minimum is about 400-420 so I think it’s good. The carburetion is already quite lean.

Do you think I have to:

  1. Make the carburetion more lean (but the spark plug looks so good with this carburetion)
  2. Try a longer ratio
  3. Change the preparation of the engine (maybe the shape of the exhaust?)


I’d say that you are too lean because your RPMs are rising and the EGT remains flat.

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I concur with Tony. Usually falling off like that is a sign of being lean. In this case it looks subtle though. How does the piston crown look?

Ideally I like to look at crown and exhaust flange. Plug color can be tempered by its heat grade, engine temp and how hard it was driven.

EGT can be kind ambiguous at times too and you kind have to figure out what works for your own particular engine.

One concept I like is that it’s hard to blow something up by going richer. If you have air density readings from the day, go a point larger on the main next day at the track and see how it responds. You could raise the needle too and see how it behaves.

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There is a possibility where the carburetion is lean while the EGT is not high? I thought that high temp = more lean and low temp = more rich. It’s strange for me to think that if I go less lean the EGT can go higher. I’m not very good to understand carburetion from the piston…I prefer to use telemetry data and my feelings while driving.

I’ll try to be more rich and see how the EGT moves. Usually I do the carburetion with the mixture throttle valve on the steering wheel and sometimes moving the pin, do you think it’s enough or I have to change also the atomizer?

I believe your thinking is correct to a point, but I was led to believe that as RPM rise you need fuel to make more fire (you are pumping more air at higher RPM), less fuel is less fire so the EGT temp goes down. As you enter the ideal setting the temp will go up with RPM. BUT fuel can also be cooling so too much fuel would cool the fire on the rich end.

Can you post a photo of which EGT probe you are using? Some will read higher/lower than others.

Also what are your current carb settings.

To me the graph does look rich, but I’m with James on everything he said. It’s easy and safe to jet up a couple sizes on the main jet and see how the motor performs. If the motor was previously too lean then you’ll notice more punch in the mid-range after going richer.

Using the return regulator to control jetting is going to have minimal efficacy. It’s a good tool to have as a safe guard, but if you need to make meaningful jetting changes (especially for WOT) then you’ll have to do so via the main jet/needle/clip/atomizer.

If it was my motor I would first adjust the carb richer like all above said. I dont recall anything good ever happened to my motors if the egt dropped on a long straight

Detonation lowers egt numbers

Egt is dependant on your compression ratio, squish clearance and timing. One shouldnt aim for a target egt temperature - it might be impossible with your current engine set up. Have a bit different squish, comp or timing and you cant reach the same egt numbers as your fellow competitor that uses the same engine platform.

Change squish, compression, timing and you can have what ever egt numbers you want. Stone cold motor that never heats up or a aluminum melting pot

Tight squish and high compression head will result in lower egt numbers

Tight squish 0.6 or so with a high comp head and I couldnt get egt over 600C with a 175sse

Large squish gap 1.3mm and its was easy to go over 700C with a 175sse

The more early or efficient the combustion, higher the pressure drop, the less theres heat going into the exhaust pipe and into the egt sensor.

Poor combustion process = large squish gap, low compression and one will have end gases burning in the cylinder - after the main combustion event around the spark plug. End gases just heat the piston, cylinder, exhaust and produce little cranking pressure

Lean vs. Rich and EGT…

You’re partly correct that if you are lean, you will generally read a higher EGT than if you are rich, all things considered.

BUT… if your EGT starts dropping at high rpm’s, it is a sign that you are probably too lean at the top rpm for the track that you are racing on. The way I think of it is that when that happens, you are not getting enough fuel to keep the EGT high.

I had it explained to me by thinking of it as a fire, and the biggest and hottest it will get is when you have the perfect mix of air and fuel… in the case of a fire, if you have too much air (lean), the fire will die down and lose heat. In a kart, when you’re accelerating, you are pumping more and more air through the motor, while the available fuel supply stays pretty consistent for the jet you’re on. That means that as you go faster, and the EGT drops, it has to be because there is now too much air (not enough fuel) being pumped through (lean).

I don’t have with me the kart so I can’t take photos, but I can tell you that the EGT probe is the one with the Aim Mychron 5.

To be honest, i don’t know the carb settings. I’m a driver and I’m pretty competent about chassis settings, the engine for me is a black hole :smile:

All about engine is in my preparer hands (that is not my mechanic), and he says that seeing the spark plug and the piston the carburetion looks good, but this EGT drop on the final metres of the straights makes me some doubts.

As @Mikkometalli explains, EGT depends by multiples factors and not only by lean or rich carburetion as I tought in my ignorance.

This is a very clear and simply explanation for somebody who doesn’t know nothing about engines like me :smile:
I will certainly try to go richer and see what happens.

The only thing that stop me about going richer is that I love lean engine on low ranges…I need a very responsive engine as I touch the gas, like a crazy horse, and I hate when is rich and it begin to stutter and grumble. How can I have a richer carb on high without changing it on low? (Yes, it’s my preparer work but I’d like to know also yours thoughts)

Transition jetting as it’s called is typically handled by adjusting the needle and clip
You can find some reference material and PDF guides on the VHSH30 here:


This is a term/description that is new to me. Transition Jetting. TIL.

Tip-in is another term used for it, where you get back on the throttle. Although I’ve only seen the top-in term used with EFI systems vs carbs.


Aim has a few different probes, all of which give slightly different readings. There is a straight probe that reads around 150* F lower than the angled probe.

Have you reached those target EGT numbers at that track with that engine before? Are you lacking straightaway speed compared to others on the same day?

Interesting, I have to check my probe and I’ll post a photo if you can help me to understand which probe I have (it will take about 15 days to do this)

This was my first time on this track, to be honest the speed was good compared to others. This is why I’m asking about the EGT and I want to understand the reasons of this drop while RPM and speed was optimal.

I noted also that both Saturday and Sunday the drop was more significant when I ran in hot hours (about 30°). When I ran in early morning or in the afternoon, when the temperature was about 22-24°, the drop is restricted to few last metres and it’s more flat. But also in this case the max EGT i reached is 605°

Okay, first off different probes will return different measures. Understand which probe you have to know what measures you should expect. Secondly, the stopwatch tells you more than the measures. Were you quicker at this jetting than that jetting. Thirdly, yes you can stick a motor being too lean and having that great snap coming back onto throttle. Transition jetting is a thing and can effect how your throttle application changes acceleration. Fourthly, depending on your Rule Set, you may or may not be allowed to change your jetting outside of the main jet. Tread lightly in this area or you may get disqualified for making illegal changes.

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