We did the same last season. used the old mini engine- removed the restrictors and re-jetted. Found the jetting and needle more sensistive in Jnr. Our engine builder said that the minimax is built for bottom end power and we might struggle to get very high revs >13K. we can hit 13.3K with a tow, but we didn't race at a high revving circuit.You might want to consider a 12.5 carb in jnr. we are happy to run our engine and get it adjusted with it's next service.A good engine is a good engine and the difference is marginal (3-4 10th's).Our biggest problem was getting up to weight adding 11 kilos of lead.I think the 13kg gap between class's is too wide and it should go back down to 145Kg jnr max( as with kfJ and Jr kpg) My advice- save your money and leave the engine as it is and let the driver get used to the class. Resell prices for rotax seem quite low at the moment.If you do sell the mini engine then Jnr Iame x30, and Jnr KPG are also options. High revs won't damage the engine Rotax JNR is designed to run at these revs and are restricted at 14K ish.
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