It's a long shot but relevant.
If you take the carb apart, including removal of the ventury (leave the stripped down carb body in the oven - when the missus is out - at about 100C - leather gloves - press the ventury out with your fingers, it should drop out if not damaged or forced in before. Below it between the carb body and the ventury is a little rubber gasket which sits around the atomiser bush and also seals around two tiny holes from the carb body to the ventury. These two holes are 'initial acceleration holes' and when the throttle is applied a squirt of extra fuel runs through them. The holes in the rubber gasket tend to to or can not line up well with the holes in the carb body and ventury. This can be to a poorly manufactured gasket - the holes tend to be out of line, or more likely poor carb build - knocking the gasket to one side when installing the ventury back (ventury in the frezzer, carb body in the oven 100C they will drop together if you are quick before the temp stabalises between the two). More recently Rotax / dellorto provide modified gaskets with bigger holes to overcome this problem. These new gaskets came in around 2012 so any older will have the 'poor' gaskets. Another option is to take a leather hole punch - plier type and punch bigger holes in the exsisting gasket. BIG CAREFUL because this is MSA illegal as you are modifying a standard component - total rubbish - not allowed to post a stronger word! as all you are doing is getting a poor component which was officially later modified by the supplier to work properly without having to pay through the nose to buy new when they supplied poor in the first place. BUT very sharp scruits will spot this even though it is difficult to spot, they know to look for it and they would be correct excluding because of it - they enforce the rules even if the rules are rubbish - there's that word again! There was a big kick off over a number of Juniors being excluded at Shenny I think for this a while ago. These holes differentiate the 8.5 to the 12.5 carb; the 12.5 has a bigger hole, which is not always so good for mini as the engine doesn't react to fueling the same way a senior might. Hence 8.5 tends to be better for mini but there are no hard and fast rules - also careful throttle application is important in mini - not too sharp and flat out as per cadet or X30 as too much fuel is pumped into the engine from the holes noted above and the airflow through the carb slows down too much to start with and then picks up.
So in summary it could be poor carb build, gasket out of place, holes in gasket too small and out of position, or holes in carb and ventury blocked - ultrasonic clean, new gasket will deal with this, or an 8.5 might be better than a 12.5 (we have an awesome 8.5 with pedigree we used in mini an broke lap record and won club champs on, but not so good on Junior and retired now) or driver needs to be a bit less lead footed coming out of corners until the engine picks up and with the carb restrictor in mid to high revs will be OK with flat out on the throttle. Just thought, does you carb have a mini restrictor in it? No performance advantage with it out but much more difficult to drive on the throttle at lower revs without the driver knowing the magic old throttle lift off to go faster by making a smaller ventury opening to speed up airflow through the carb to get it to work properly with the mini engine. My lad spent most of one season picking this up and then was very good at it, then then brought the restrictor in which was good for mini racing but we lost an advantage on how to work the lift off!!
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Rotax Mini bogging on exit of high speed corner by '1509' (22 Sep 2014 @ 8:59)
