Seems to me that air cooling is lighter, has no point of failure (forget to put coolant in, hose pops off mid race, pump fail, etc).
It seems that all the current 100cc are air cooled. It also seems all of the current 125 are all water cooled.
I’m guessing there’s a reason 125 is water cooled. What is it? Does the higher power of the 125 require that much more cooling than 100?
There’s 125 aircooled motors (old Sudams, the various iterations of Easykart) and 100cc water cooled. The CIK 100cc classes (the proper ones) went waters around the 2000 era, and that was kinda a matter of reliability/noise issues the CIK was trying to lessen.
The KAs came out because of Australia wanting an alternative to the Yamaha KT100 and IAME basically put a clutch on a Reedjet from the 90s and detuned it. The governing body, iirc, tendered for it and already had rough specs they wanted.
Aircooled engines, generally, have a lower cost to purchase, but are louder and have more issues with heat. It’s not really a 100cc vs 125cc thing, it’s really dependent on markets and governing bodies at any given period. It’s just the way it’s shaken out we see what we see today. There was a time when all ‘100cc’ engines were watercooled (FA/ICA circa 2000-2006) and a lot of 125cc engines (EasyKart in particular) were aircooled.
Karting Australia went 100cc because it has less vibration and doesn’t need a balance shaft - IAME KA100 TO REVOLUTIONISE AUSTRALIAN KARTING FROM 2015 maybe packaging a balance shaft on a aircooled is a bit trickier. I dunno.
We used to have the KFS125 (IAME Komet 125) in France. 30hp, same lap times as X30/Rotax Max but not as reliable
The larger the piston, the harder it is to cool with passive air flow. Water cooling easily solves that and keeps the package size of the engine small.
I’m a big fan of aircoooed for the sound and not having to deal with extra stuff to bolt on and check…
In general not as reliable in higher power output two strokes. Over the course of ownership more prone to siezing and rebuild period tends to be more frequent due to increased cylinder distortion.
Noise regulations kinda multiplied it in some cases.
Key factor here: Higher power output applications, shifters, formula A etc. If you keep the power down they can work well, especially with modern tooling and machining.
VM did have an aircooled 125 recently, but I don’t think that’s true anymore.
I think saying they detuned it might be a bit of an understatement. The castings are similar but the bore, stoke and reed dimensions are different in the KA
85% of the 2 stroke field at my local are all on 125cc Air cooled easy karts. They are justing starting to get a few X-30’s in the mix.
That’s awesome your local track seems to have some unique classes.
EasyKart was hot in Florida for a while, I’m sure there are plenty sitting around for cheap. Would be a fun engine. A Leopard without some of the complications.
I reckon the engine package of EasyKart alone would have been a huge success. Marrying the engine to the chassis as a concept seems to limit success. For whatever reason, despite the market being heavily single-spec on the engine/tyre front, we still want to choose our chassis.
Wasn’t the Easy Kart platform the 2 stroke version of the spec classes like L0206, except with them you had to run the same motor and chassis? I feel like I’d heard that from somebody who runs those things, but maybe I’m wrong. 
I’m not sure there’s any parallel between 206 and EasyKart, other than them both being spec-engine classes. But most classes are spec-engine today. EasyKart was IAME’s answer to a Rotax-style international spec-engine ruleset, but yeah the chassis was also a spec Birel chassis.
EasyKart was more like the Tillotson spec series or Margay Ignite in the sense that the engine and chassis were spec.
Makes sense, my local has always been a Birel and IAME dealer. I think the 2 stroke field isn’t tradition, as most are easy karts, but they allow anyone on a 125cc into that class, IAME, Honda, Rotax I believe. (Not sure on that last one) Easy Kart, X-30, and Honda for sure though- those guys run in the same class.
I see Easykart as an attempt by Birel to corner parts of the market exclusively to themselves.They supply spec chassis through their agents and then get IAME to supply versions of obsolete engines solely for Easykart.
In UK Easykart is completely run by the Birel importer. Spec.chassis.Spec sealed engines.Spares ,Rebuilds, The lot even to the organisation running the races.Not saying this is necessarily a bad thing but that’s how it is UK at least.
What makes air cooled sound different than water cooled?
A water jacket is a good sound insulator whereas the fins on an aircooled engine are kinda like a loudspeaker 
