Some theory on kart tuning and driving (check "Kart Steering, Physical Forces and Setup - Theory and Practice", "Driving in the wet" and "Overtaking"):
http://www.karting.co.uk/KandK/Tech/index.html
As for tyre pressure, start with the middle range of the manufacturer's recommendation. That's normally not good enough, because what I'm watching right now is a recommendation that for 11 x 6.00-5 you should use 11 to 23 psi, which is a wide margin! BTW, kart tyres have a different nomenclature from your car: 11 is the tyre diameter, 6.00 is the width of the tyre and 5 is the size of the rim, all in inches. Anyway, start at around 16 psi.
You shouldn't start to "fool around" with settings until you are able to get 0.2 seconds differences, maximum, between laps for, I don't know, at least 8 laps.
If you get good lap times on the first three laps and then your times get worse, you started with a good pressure but then the tyres heated and got overpressurized. Diminish pressure in this case. What you shoud get is that you start getting better times in the first three laps as the tyres heat.
Lower the pressure when the day is hot, the race is long or there is rubber on the track. When you lower the pressure you decrease the load on the tyre and generate less heat on the tyre. That has a limit: under 10 psi or so, lateral walls flex a lot and then you have heating of the tyres again.
Inflate a little when the track is wet. Inflate about 3 psi when using hard SL tyres.
When the track is drying, it is important use low pressures in wet tyres to avoid overheating them. If you can, use an old wet set because they are hardened and will give you a little more grip on a track that is drying. Of course, the best is to change to slicks as soon as possible, but you know how the things are when you start to trust your instinct about if rain is coming or going, specially on long races...
If you increase rear pressure by, I don't know, 1 to 3 psi, you will get a looser rear end (more oversteering). The same applies to understeer: lower the rear pressure by the same amount. It's easy for a beginner to confuse oversteering with understeering (you think that the kart is oversteering, but in fact you are getting a loose front end and it "transmits" the instability to the rear).
If you cannot fix some problem (specially when the rear end does not "slide" properly) you can try wider rims to distribute the load better. On circuits and frames that cause understeering, large front tires can be used OR smaller rim size.
Finally, tyre diameter varies with speed and pressure. This must be taken in account: change the gear ratios when you think this is happening (if you change "too much" the pressure, for example, or for a fast track). I'd say to change the gear number for each half an inch of variation in the
circumference of the tyre.
That sums up most of what I know. If you need a hand, let me know. I have a page on kart tracks that I haven't published yet. If you wish it in its current state (not checked entirely) I'll be glad to provide it to you. If you have kart track locations, thanks. A final tip: listen to others and
check for yourself everything they say. Karts teach you to follow your mind and only yours. You can become a team player when you move to faster categories...
