That will adjust your CO so ideally you need an exhaust gas analyser to set it up properly. If its out in the first place, then it will effect your idle. What ever you do, don't rev the engine whilst the allan key is still in there............you'll be looking at a new metering unit otherwise

There is a special tool but all it is, is a 'T' bar allan key, so if you have a long enough allan key, that will suffice. I thought it was 3mm but I may be wrong..............