Before you start messing about with any setup you need to be a consistent driver. In a Kosmic/Tony Kart/ Alonso chassis the sweet spot for setup is quite wide, unlike CRG's and some other chassis which have a much narrower sweet spot in my opinion. So if you are just starting out. Just run with what you have and put down some serious laps and time in the seat. Mike and Clay kart shop that churns out champions of Clay and Dunkeswell on a monotonous basis says "its just bump in seat time that matters". Meaning if you have any sort of talent at all then practice will make you fast.
That said once you get to the point that you are consistent, but want those last tenths, its time to start getting down to some nitty gritty setup.
I am going to talk MiniMax and Junior Rotax because that is what I know and what we won championships in. MiniMax is about maximising corner speed and ensuring the kart doesn't bog down mid corner because you just don't have the pull to over come the grip you have. We ran with MXP and MXJ wheels. These are half mag wheels and I don't actually think it is essential to run full mags for MiniMax. The power and weight don't warrant it. I know people that did though.
It you have £500 to burn on a set of Mags then go for it. Don't buy the Douglas ones though they just don't work. I don't why but they don't. Mike at Clay kart shop did find an alternative to the expensive OTK Mags which where about half the price and he said worked the same. I didn't try them so can't but worth a ring. I managed to get hold of a set second hand (always check they are straight before you buy them by putting them on an axle and giving them a spin.
In Minimax on bumping tracks or where you need to take curb then running a softer axle can be good. OTK N is the Normal medium axle. It's actually pretty good, as I was saying earlier. You can actually soften the effect by cutting off a few cms from the each end. This gives less support to the wheel hub and allow more flex. We can cut down axles for a while and then tried an S which is a soft axle. You have to watch these axles as they can bend where the grub screws hold it in the bearings and then eventually snap off. So you need to keep checking the axle. We actually found the S a bit too soft and settled on the Q which is half way between an S and N. I would say slightly softer than a cut down N and just worked better.
Keeping an axle clean and bearings well lubed is essential. Taking it out regularly will stop it corroding in place and when your pilot ends up bending one just before the final you will be able to get it out and change it in time if it has been maintained. Yep it is one of those things you should have as a spare. If you are rubbish at changing them practice, or get help for race day. I was probably the slowest mechanic in England, but practice and care and my son only had virtually no failures on track (ok the famous back bumper bold snapping off in final when he was 20 seconds ahead, but pretty nothing else)
Junior Max, if its dry FULL MAGs end of story. They are just faster and smoother and more drivable with the extra weight and power. SO you have to get a set if you are racing Juniors. We ran with a Q axle as well and found that to be our preferred setup just about anywhere. Then its down to tyre pressures and getting the front up set up, but that is another is story.
Oh just one other thing. Width or track at the back. I saw a lot of people messing with this in the dry. It's simple, you want the back of the kart as loose as you can, so it comes off the corner as fast as possible. So it's maximum width at the back if it's dry 100% of the time. If its wet sure you can come in. The problem with an OTK kart is the wets are narrow anyway and the wheels have to be out beyond the side pods in Rotax (different for TKM) so there isn't really any room for manoeuvre to bring them in. Having a long straight edge to double check you are ok is a good idea. Also drilling a an extra hole in the sidepod bar so you can push it in further and run a narrow track is actually illegal. I've seen a lot of people doing that as well, and never seen any penalised for it.
So you should have a kart that will ride curbs and come off the corner fast. It's funny, laughable the amount of people I saw stammering "Look at that engine, did you see the way it came off that corner and shot passed my lad. God what I would do for an engine like that..." It's not the engine, your pilot duffed the corner, bogged the kart at the apex by sliding it and then it sticks kills the engines and won't drive off. Get the lines right and the set right so you can carry the speed.
Now you should have a kart and pilot that will be happy to attack and throw it up the inside and round the outside and still be able to carry the speed. Riley popping it up the inside...
I think I'll talk ride height next time, unless you have serious weight or height issues it will be fine where it is.