There are many rotax experts out there, I am not one of them as it has been many many years since we raced mini max or even rotax. However here is some basic dellorto info that may help you:-
The Carb is to big @ 34mm, (45hp KZ engines only have a 30mm carb). If the throttle is wide open at low rpm, simply put, the carb will not give the correct mixture. Therefore the throttle needs to opened progressively during acceleration which is counter intuitive for a young driver and needs to learnt.
Progression/acceleration at part throttle is controlled by the needle. changing the main jet will not help if the throttle is being operated correctly. Try both needles K98 at richest and K27 at leanest, get the driver to learn the the difference in symptoms between being to rich and to lean during acceleration, obviously the optimum is somewhere in between.
It is interesting that a slide restrictor has been introduced this year, it used to be common practice to restrict the throttle opening on mini and junior back in the day. Depending on how much the restiriction is, you may find the main jet becomes redundant!
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