My question stemmed from looking at the apps (“Jetting Max Kart” and Rotax’s official “Rotax Max Jetting”). I have a Sr. Evo (used to have DD2 non-Evo, but not the concern here). I’m trying to learn.
With the same weather condition, why do the jet sizes from the 2 apps (supposedly both for Rotax Max Evo and non-Evo) vary greatly?
Today, as an example, Jetting Max Kart is calling for 162 while Rotax Max Jetting is calling for 126.
Both (Evo and non-Evo) are 125 cc, creating roughly the same HP. Stoichiometrically, they both should roughly require the same amount of fuel, regardless of the carb.
I know that Evo and non-Evo have different carbs, but why the jet size difference?
The new carb pulls more fuel from the progression circuits so that it doesn’t depend so much on the main jet. An EVO is getting the same amount of fuel, it’s just when it’s delivered that’s changed. It’s why they had to change the timing and ECU to go along with the new carb.
That’s not true, the main reason the jetting is so different is because the new carb uses a different atomizer. The atomizer in the new carb is a proper 2 stroke specific one whereas the old carb was using a 4 stroke atomizer (that carb was never meant to be used on a 2 stroke in the first place). The ignition timing on a senior evo and pre-evo is the same.
The question was, because the EVO uses a smaller main jet, if the EVO requires less overall fuel than a Pre-EVO. The answer is clearly no; it’s just a matter of how and when the fuel is delivered. So all of the internals, including the atomizer, the needle and the carb body itself, contribute to that. And the EVO ECU absolutely has altered ignition timing. It is slight, but it was done specifically to work with the new carb and make the the jetting less sensitive in the higher rpm ranges. It’s since been updated twice since 2016.
Agreed on the using less fuel thing. I wasn’t aware the actual ignition timing curve was changed on the senior. I know there were changes with max rpm and that the junior evo definitely had a different ignition curve vs the junior pre-evo.