basically, the main thing that mechs do when they tune your
carburator is the air/fuel mixture. in this equation, fuel is the
only thing that will remain constant throughout, whether it rains,
snows, thunders, whether the sun is up or down, the amount of
petrol (and 2t) going through your carbs will forever be the
same.
AIR however, is very different. the type of air going into your
carbs changes with the temperature and humidity of the area.
today, it is bright and sunny, temperature is at a nice 35°,
humidty is at... er.... check the barometer, forgot liaoz.
your mech tune your engine for you at 2pm. the air is cooling and
less dense (not alot of moisture). tune until super kao lat kao
lat. wah u ride that time damn song.
nxt day, the weather damn kan pua hot. stand under side 2 mins only
sweat like pig liaoz. the air will be full of moisture and your
engine will find it harder to ignite the petrol. blah blah blah
blah blah.
harder to ignite = less power loh. 
