Lollipop, there is no easy way, but you could start with determining any pedigree the engine has - what has it won and by whom, then do some research on this to check it out. Don't bother with any info. from dyno readings unless you are comparing two motors tested on the SAME dyno on the SAME day, or ensure the Dyno has software that equalises out different atmospheric conditions if motors are tested on different days. All complicated stuff and difficult to confirm. Finally do not pay anything more than bottom price for a set up without testing it first. That is also not your lad testing it as he won't have the minimax experience. See if you friendly with one of the front running minimax racers at your local track and ask them if they would back to back test it for you. A rule of thumb with Rotax is; In mini you need very good bottom end power as you get no top end due to the restrictor - the area under graph on a dyno read out is more important that peak power. Carbs - mini run 8.5 best, also a top carb will be good on any engine - no need to match carb to engine. Typically more pre 2004 carbs are best but any year can be good. But again don't pay a lot for a carb without testing. Not a huge amount in exhausts in mini, but good exhausts in Junior or senior don't always tend to work well in mini. Same goes for coils - not a lot in them - later green plug coils might run smoother for mini than the earlier black plug coils. You need the post 2009 clutch assembly. Also the later airbox and Rotax marked airbox parts to run in MSA. My lad when in Mini won the club championship, and the NKF round there against 'Lando Norris' motors, and broke the lap record with an engine that read '154' on the dyno. Top mini engines are 165 on the dyno and they say as a rough guide 3 points is worth a tenth on the track, so top engines were 3 tenths faster than ours and we still got results. So as with all kart classes it isn't all in the engine - driver and kart set up is more important. There are more kart set up parameters in junior / senior karts than cadet - so kart set up is even more important in mini than cadet. Also driving style is different in mini to cadet.
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