Lean vs Rich: I try to achieve basic understanding

Lean vs rich:

We use terms around here that most understand but not everyone. I fall into the kinda understanding what stuff means category because I’ve been hanging around here a bit, but I am learning as we go. Bear in mind I never did my own wrenching and relied on Jerry to get us set up properly. Also, my beloved rental karts have no adjustments we can make as patrons.

I thought it might be helpful to do a thread with “explainers” where I explain something in simple terms and you guys correct me and elaborate.

Lean and Rich is such an example. What I’ve gathered thus far is that this refers to the fuel/air mixture that is determined by your carburetor settings via the “needles”. Depending upon how you set these needles, your air/fuel mixture could be heavy on fuel (rich) or heavy on air (lean).

Ok great, so what do we do with this knowledge?

I’m not too sure. To begin with, we should probably discuss what lean versus rich “looks” like.

Lean, to my understanding (more air/less fuel), runs hotter and produces more power in some circumstances but isn’t something you can use for long. Our fuel contains oil, so our fuel is our lubricant. If we run lean too long, we are starving our engines of lubricaton, running it too hot, and potentially seizing the piston in the cylinder. So, while lean may be zoomy, it’s too hot.

Rich, on the other hand, means more fuel is in the mix, and I am not sure what the result of rich is in terms of engine behavior. Plenty of lube but maybe potentially boggy on the bottom end? I’d imagine the plentiful fuel means that you might have incomplete detonations or something. But, there has to be some circumstance where someone would want to richen the engine.

How do you know if you are running too lean versus too rich?

Again, not sure. Someone jump in here. On track it must be in how the engine “feels” and responds to power being put down.

Visually, I have seen pictures of our piston crowns being referenced in lean vs rich discussions. It would seem that lean results in certain type of piston wear and deposits on crown versus rich and can be identified visually.

Edit: Adam contributes:

If I have my kart setup up fine, will my carb setting ever need to change? Will I somehow go from correct settings to lean or rich?

I think so. What seems to change wether you lean out or richen the mix is atmospheric conditions and elevation. As weather/temp/elevation changes, so too does the process of combustion and you need to make adjustments to carb needles to compensate.

And that’s all I got. Fire away. Please feel free to hop in and clarify/correct/elaborate.

The intent of this is to try to de-mystify a lot of the jargon we use for non wrench folks and newbies. Like me. :sunglasses:


Great idea for a series of threads. As a noob myself, one thing I have noticed about karting is that nothing (except the engine) comes with manuals or directions, and there is a lot of wonky lingo. I’m mechanically inclined and I enjoy making stuff and wrenching, so I’m muddling through, but I’m sure thankful for places like this. Mechanical lingo aside, the dizzying variety of acronyms for kart types, manufacturers, and classes can make it difficult to understand what you’re reading.

My $.02 on this topic, drawn from general engine experience is that you want to run at the mixture that makes the best power without running too hot. In my experience, once you find the right setting, you only need to vary the needle settings a tiny bit, if at all, to account for atmospheric variance. Since I’m not competing in anything yet, and have no plans beyond club racing for fun, my approach is to stay conservative and not run too lean. I haven’t adjusted my needles at all from one day to the next. My plug looks good.

Too lean leaves your plug white and chalky. Too rich leaves your plug fouled and oily. It also spits more oil out of the muffler.

Thanks! Feel free to jump in and ask/discuss whatever you think might be helpful!

If anyone can address how a kart that runs too rich or too lean “feels” different, that would be helpful.

Edit: TJ adds:

It really depends on the engine and carb setup. Different carb configurations might feel different in how they react to too lean or too rich.

Generally, if a carb is set too on the low end rich, it will be “boggy”, or feel like it’s lugging a bit off the corner. Too rich on the high can manifest in different ways. Some engines will “4-cycle” (sound like a 4-stroke engine) on the top end if they are too rich. So you’ll notice a pronounced engine note difference. Too lean generally feels like the engine momentarily cuts power a bit as it runs out of fuel. Something like a Rotax might pop on the top end a bit if it’s too lean.

But it can vary. On the KA, those feelings can be sometimes reversed. Too lean on the bottom can make it feel boggy.

I would worry less about the feel of it and more focus on the EGT and reading the exhaust manifold color or spark plug color. Once you get really in tune with the kart and perceptive to the differences in how those carb conditions feel, you might be able to tweak the carb a bit on-track to really fine tune it, using EGT as a reference.

If you want to know how to tune the carb using EGT, go back and read some of Nunley’s posts. He explained it better than I ever could.


Thanks TJ.

I have experienced “four-stroking”. When I went to test the 100cc at DKC, it wasn’t hitting the top end properly. I brought it back to pits and Mike said it was “four stroking”. He hopped in the kart and proceeded to tear around in concentric circles while fiddling with the needles. Once he was done, it ran fine and hit top end and sounded normal.

Edit: TJ brings up Al and EGT, perhaps this thread:

Yamahas really did it bad, but I’ve heard it in KA as well.

Great topic! I’ve moved it to 2T simple because they are more sensitive to jetting\carburation changes.

Ok I’m gonna go out on a ledge here and suggest that heat and lean are closely associated and that it’s pretty likely that in hot weather you may need to richen?

Ergo, conversely, when James goes Ice-racing in Minnesota, likely needs to lean?

Or does none of that matter once engine is warmed up?

Weather matters technically.

Does it change things significantly enough that you fiddle with needles regularly?

The problem is that rich and lean that they are very subjective/relative terms.

Generally you want to manage heat and jetting separately. That’s to say, you should be able to run optimal jetting for performance without having to worry about cooling. But older aircooled two strokes (KT100, ICA\FA\JICA engines etc) are fairly undercooled so jetting is used as a means to control the temps.

We jet the karts up (Both pilot and Main) in winter because the air density increases, sometimes up to 105ADR, plus more fuel condenses on the intake tract walls in cold temps so there’s some compensation for that I guess. Our summer jetting does not go well in winter and vice versa.

Temps wise we have to cover about 90% of the recoil vents to get things up to temp and keep them there. We also need that blockage to stop snow lodging in the recoil mechanism. I shoot for 250F head temp in testing to allow for days when it gets to into the 30’s.

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You’ll typically find around a 4 minute range that needs to be adjusted in for weather on a KA, not sure about an X30. In the rain you lean out the carb even more since there’s so much water in the air and with a rain cover you’re reducing the airflow into the carb.

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Aaron, by 4 minutes do you mean that one full turn of the needle = 1 min? So 4 mins would potentially mean 4 full rotations or am I completely misunderstanding?

That seems like I’m misunderstanding. Maybe there are 60 minutes in the hour (full rotation of needle 360 deg) and thus 4 mins is a small tweak.


Minutes means the second part. 1 minute is 1/60 of a rotation, 60 minutes = 1 full rotation. With my drivers engine builder this past year, we had 2 engines. One had a range of 56 minutes for lean carb settings, 60 minutes for high carb settings, the other was 58 minutes for lean, 62 minutes for rich.

Carb also is effected by the gearing. You can run less teeth on the kart and richen up the high end to make up for it (I’m not sure exactly how that works but it does). We ended up 6 or 7 teeth lower than our other driver in KA Junior at one point and ran the carb at roughly 75 minutes. Way richer than we would normally but that allowed the motor to pull out of the corner and still have top end speed.


Mind blown. :exploding_head: That’s wild.

The wild part was that it worked. We figured this out at a regional race at my home track, a small technical bullring where the gearing is generally like a 10/78 or so in Junior. I swapped over to the race motor but didn’t realize I hadn’t switched the driver from an 11, so we ended up on an 11/78 or so instead. Couldn’t figure out the carb, spark plug looked lean but exhaust looked rich, so eventually we opened up the high needle a quarter turn and everything ran better. We were the fastest kart the whole weekend which was the craziest part, against a lot of local drivers.

Go to GoPro the next weekend, swap motors still thinking it’s on a 10 tooth instead of 11 (I even counted and missed a tooth or something) and the motor was doing the same thing, but looked sluggish out of T8. Eventually we figure out I messed up, put the correct gearing on, and my driver starts making a bunch of mistakes. I end up dropping 3 or 4 teeth, adjust the carb accordingly, and don’t tell him until he’s on the grid about to roll out. He fixed all the mistakes because he knows the gearing won’t cover up a mistake anymore.

This is a lot of why I want to learn how to build an engine, because our engine builder knew how the gearing and carb would work together, which is why we dropped 4 teeth from the “expected” gear ratio after fixing everything.

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Good stuff and thank you for sharing. Totally non-intuitive.

The faster you go!

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Valuable topic that’s qualitative mainly in our sport. For sake of a quantitative example I’ll share a recent AFR adjustment.

For reference, I’m a “get the laps in” Karter that happens to be a data nerd and not a hardcore racer subject to a ruleset limiting what I can have on a kart. My karts are basically rolling data labs.

Two screen grabs, this is from a Lo206 at sea level in Ca. As temps drop air density increases and summer carb settings become Leaner than ideal.

The best way to know is through a Lambda sensor, which provides values relative to Stoich. Above 1.0 being LEAN and below RICH.

The difference between these two sessions was a float height adjustment of about .015" from .835 to .820". The engine did pull better, especially off the bottom end and seemed to pull better than other karts on track.

First image is with the corrected float height and pulling a .88 Lambda sustained WOT, arguably ideal, but not verified on a Dyno to know where max HP is relative to AFR. I don’t have dyno access unfortunately. The Red is a channel alarm to quickly see anywhere Leaner than 1.0 Lambda which indicates that all the fuel was burned in combustion and there is excess oxygen in the exhaust which is lean.

Keep in mind AFR readings that are not open throttle are basically irrelevant, we’re concerned about AFR under load, sustained open throttle.

This image you see lean AFR / LAMBDA


Jeez… 11 degree diff between FR and FL. Seems big. Ignore my off topic digression.

So the purple line is .88 lambda top and 1.01 bottom.

Column 18 seems to indicate that running the .88 line results in greater speed and rpm. Or, am I reading column 18 wrong and it’s not an average of the lap?

While not specific to rich and lean, I think its important to understand what is going on with the carb first.

There are two videos that helped me understand carburetors and what is going on with them. This one is a 4 cycle carb that Dustin from Smarter Every Day 3D printed to “see” what is happening. Very interesting:

The second one is done by a guy that has several videos on the subject. He calls himself The Repair Specialist and he explains 2 stroke carburetors in great detail.

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