Street Legal 2 Strokes and Jetting vs Karts

Had a thought after reading “Stealing Speed”, a true story about the development of 2 stroke technology. The Cold War era led to a lot of 2 stroke engines within the Iron Curtain: cheaper to manufacture being a big reason. Now, having had way too many street legal 2 stroke motorcycles, it made me wonder: Bloc nations ran 2 stroke cars, scooters and motorcycles constantly. I had a few 2 stroke sport bikes and jetting was only a concern when fitting an aftermarket expansion chamber/pipe. Why is jetting with karts a constant concern? In my head, I think “maximum performance coupled with being run WOT” which stresses the engine. In theory, couldn’t a kart be set up with a “middle of the road” jetting configuration to cover “most” environments, similar to what most OEMs did with carburetors prior to the switch to EFI?

You’ve basically answered your own question. A transport vehicle doesn’t need to extract maximum performance from the engine, so it can be jetted & tuned more conservatively.

So then if someone wasn’t chasing maximum performance, they could conceivably jet conservatively and “set it and forget it”. I’m curious why this isn’t pitched more often in the various books on entering karting just to minimize the various stressors for new folks.

Karting has many engine packages so it depends on what engine you’re looking at.

Something like a KA is pretty easy to set up and doesn’t need much tinkering. You could set it pretty conservative and not give up much and barely touch it.

Something like a KZ or Rotax is going to be less forgiving and requiring more precision.

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Depends. I think the pursuit of less stressors isn’t necessarily a good thing, even for newcomers. Actually I’d go as far as saying the aversion of a steep learning curve has damaged karting long term. It’s good to have a sport that has some depth to it. The more more throwaway you make something the more people end up … throwing it away. I know that’s a speculative, but to some degree the data backs that up, well if I cherry pick it.

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+1 on that. Also from a technical standpoint, kart engines have more extreme port timing vs a street engine, so jetting window is also narrower. The more extreme engine platform or package you are running, the tighter the window. That for sure forces you to learn jetting, which is a good thing (and gives you the cheapest power advantage money can buy)

I’ve run two strokes out of the racetrack (mostly Aprilia) and you could see it immediately. Stock you’d just change max jet in fall and spring and that was it. As soon as you started modifying them, things changed really quickly for those engines too

@Alan_Dove understood and agreed, in my mind (which is severely impacted and constrained by NSFW language) I had thought “if you had the ability to make karting a bite-sized piece pursuit in bring people in, what would I do?” and had thought removing the mechanical aspect of the sport in the beginning and focusing on understanding racecraft, flags etc. and then bringing the technical component (which I agree is a critical piece of this sport and has the participant invest more time and mental acumen which makes the sport “stickier” for interest) would at least allow someone to decide if ownership or arrive-and-drive/100% tented is more for them.

@Andy_DiGiusto RS250? Always wanted one, had a MB5, RGV250 and a TZ125. Still waiting for the holy grail NSR500 or RG500 Gamma to appear in my garage.

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Honestly, no-wrenching arrive and drive is heaven for some. I kind of like it for easy race days.


AF1 125 Reggiani Replica…at age 16, I was too young for the rs250, that required to be 18 or 21 I don’t remember exactly. I got that bike to 160cc with a special cylinder kit, special ignition, removed mix system and put a race carb and exhaust. Super light, super fast. Those were the golden years, we just didn’t realize it it back then.
RGV gamma 500 was (and still is) a dream.

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People certainly do this. Look at the average shifter kart out there - it’s a 15ish year old chassis with a CR125, a pipe that was the hot setup a long time ago, and a nice fat main jet that’s fine on all but the coldest day.

The disadvantage is being passed by X30 drivers who are turning the needles on track to get their target EGT!

Depends. There’s plenty of depth already just in trying to figure out chassis tuning. Could even argue that 1 man’s engine jetting “depth” is another’s high maintenance hassle. How many people avoid diving in because they don’t want to deal with the “black art” of jetting &/or worrying about their motor grenading? I created a thread about applying the Smart Carb to a KZ engine for my KZ-ES that would essentially yield EFI style performance in a carburetor package. Is that a “bad” thing? Doubt. Improvement comes in many forms. Imagine having more time freed up to dial your chassis & getting on the track & not having to worry about your engine’s performance & reliability. Imagine how shallow this sport will become if it ever goes EV…

Not happening. The Big 4 derive most of their revenue from selling parts, as opposed to unit sales. Honda in particular pushed the FIM to migrate to 4 strokes because the GP class was a uniquely exotic “race circus” like F1, & there’s a greater degree of product equivalency with the 4 stroke bikes to enhance showroom sales (& by extension, parts). The Bimota V Due was the potential nexus for 2 stroke street bikes, but it was plagued with problems related to the electronic DEFI that gave the concept a bad rep, & after they went back to carburetion to resolve it, the legal EFI 2 stroke street bike concept withered. I’ve heard that the core problem turned out to be bad case sealing & not the actual fuel mapping, but done is done. A 2 stroke V-4 sport bike with counter rotating cranks would’ve been cool, though. As it stands, the sport bike market in general looks to becoming a dwindling niche, especially since the 600’s died off.

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Less stuff to talk and converse with people about on forums etc… It basically rips the culture out of the sport. It’s not worrying, it’s just part of the sport. And thus it isn’t simplification but destruction of the sporting contest. What it does do, imo, is make karting easier to avoid do something that does the ‘not worry about engine reliability etc…’ far far far better… sims. If karting is apologetic about what I believe are features and not bugs, then we are giving our opponents and open goal. By saying jetting is complicated and difficult, rather than part of the rich sporting contest, we are ceding position to our opponents in the market place.

It’s a fine balance of course. Complexity for complexities sake isn’t good. I think surface level simplicity with plenty of sub-level complexity and learning curve is good.

But once we remove the areas of the sport that give it depth, we lose so much, which has happened to a large extent. The kart media outlets have all but vanished in the UK and somewhat in Europe. Without that, there’s less inherent promotion and so on. These things happen of course, but I think are symptomatic of wider problems. I will say it’s all too far gone to change now though.

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I don’t think jetting should be a “dark art”, but sadly sometimes is perceived that way. In my opinion, a well rounded racer should understand engine, chassis, tires, driving, racecraft etc. You can’t remove one part of the equation. Sure you can pay someone to do it for you, but who’s giving the feedback? Ask the top drivers in KZ or X30 or KA, they know exactly what jetting they are running, what changes to make and why, learn and adapt all the time and give feedback to the tuners in attempt to stay ahead.
That skillset is built over time and part of the journey, like everything else (tire pressures, chassis setup etc). And for the average hobbyist not interested in national level racing, it’s not that complex either, so my question would be…why is this aspect such a big deal? Fear to granade the engine or lack of learning/training opportunities? Now we also have apps that tell you what to do…lack of trust? I’m truly asking as I’m having hard time figuring this one out

Without going too far down a “what if?” rabbit hole, I’ve noticed so many parts of our lives are “softened” due to technology, and not necessarily a bad thing. Karting: lap transponders vs. having someone manually score/time with stop watches.

I was speaking to a team owner and he was saying this winter has been insane with people looking for chassis/engine packages, new or used but RTR. He’s never had so many unit sales before. It got me thinking about how this may be a result of post-covid “i’ve been cooped up and want to try…” or a result of “Drive To Survive” or maybe even just a “YOLO” mindset, who knows. But if let’s say 15 people bought new/used packages from him, how many people are ready to jump in with both feet and truly understand what they are getting in to? Unless you simply want ownership but with arrive-and-drive convenience via full-tent service, there’s a lot to learn regarding what the chassis is telling you, what the engine is doing, why reading a tire will tell you etc. When you think about it, IMO, its immersing yourself in engineering, understanding physics, chemistry and mathematics. Not just jumping in a pleasure kart and going on your merry way (if you want to develop as a racer). Of those 15, how many are going to stick with it or is it a passing fad that will fizzle out once the novelty wears off?

Just the result of an idle mind when we’re so close to the season starting but its still so far away. This will be my first full season and I think its pre-season jitters combined with too much empty space in my head.

Well said Alan, Well said.

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Alex, your point is absolutely spot on. It does take quite a bit of time to gather all the necessary information in order to make informed judgements about setting up karts. I would guess that many youngsters who depend on their adult parents or grandparents get frustrated and quit the sport before the adults gain the necessary knowledge to be competitive. For that reason I wrote a book: “The Lil’ Green Book of Karting”. I wanted beginners who are looking for fundamentals (and as you say it is engineering, physics, chemistry and math) to have a resource. These karts, engines and tires go through the homologation process and therefore they are engineered to be “safe and consistent” in operation and setup. It required lots of research and interpretation and I’m quite sure most wouldn’t go there. Now, I could be criticized for reveal “secret sauce” information, in a manner of speaking, however, I’m more interested in getting more people involved and competitive quickly.


@Iceman17001 the book sits on my nightstand as a frequent “go-to”…and it is a wonderful tribute to your brother.

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Glad its helping you get up to speed!