Ok, first off, close off your main needle all the way (not too tight, we don't wanna stuff anything) and then open it out 2 turns. This should be pretty close, but you'll have to check it out on the track to get it 100%
Take off your air filter and look at the opening inside the carburettor, it should be about .5mm to .75mm, set it to this by adjusting the idle screw...this is almost a once only adjustment, basically if your motor wont idle with this setting then more than likely your bottom end mixture is set too rich or too lean.
Ok, now the tricky one, how do you know when your bottom end mixture is set correctly?
The way I test it is like this.
Start the motor and get it to operating temperature, give it 2 or 3 good revs and listen to it as it comes back to idle, don't blip the throttle to keep it running, just try to hear what it does for the next 5 - 10 seconds after you rev it.
If it sort of idles, then progressively begins to idle slower and slower until it stops,then your bottom end is too rich, a good way to test this ,again is to give it 2 or 3 revs, let it come back to idle and just as it does this, get some pointy nosed pliers and clamp the fuel line about 10mm from where it goes connected to the carby. Now listen to the motor, if the bottom end is close to being right ,it should idle for about 2 to 3 seconds, then the revs will rise and the motor will stop. If it just stops as soon as you clamp the hose, then the bottom end is too lean , if it idles for longer than 5 seconds before the revs go up , then the bottom end mixture is too rich.
When adjusting the bottom end, move the needle about 30deg at a time, and don't forget to give it a few revs to both clear and to let the motor "settle" back to idling before adjusting it. You might find that once you have the mixture set correctly you may have to give the idle screw one last tweak to get it just right.
What I look for mixture wise is that after a good rev, the motor comes back to idle, then maybe ever so slightly the revs drop. Also when I hold the throttle flat down the longest straight, you should see a slight trail of smoke.
Don't worry yourself too much with the engine temp (I know, everybody says " don't run any hotter than...") you'll have to trust me on this,My GS15R's have run as hot as 300F, I've found that they don't really make good power under 280F, but I seldom look at the temp, just make sure the thing has a little trail of smoke when it's going flat out, both cool power and hot stuff use very good lubricants, you won't have any problems.
Also, when you use a higher content of nitro, you will find that the motor is easier to tune, will run cooler (to an extent), will idle better and make more power.
Courtesy of Lue.