Jetting is the dark art that isn't. The carburetor is a metering machine, the jets are its settings, and the manual's chart is where every tuning session starts — because a lean engine is a dead engine, and folklore kills more minis than wear does.
What jets do
A carb meters fuel through fixed orifices at different throttle openings: the pilot circuit handles closed-to-quarter throttle (idle, tips into corners), the needle and its clip position handle the midrange, the main jet handles full throttle. "Jetting" a bike is choosing those settings — and the reason it is a constant conversation is that the correct choice depends on the air: temperature, humidity, altitude. Cold dense air needs more fuel; hot thin air needs less. The engine that was perfect in May's cool morning is lean in July's heat wave at the same track — not because anything broke, because the air changed.
Lean is dead
The asymmetry to tattoo on the toolbox: rich is a symptom, lean is a funeral. Rich sounds blubbery, fouls plugs, smokes — annoying, cheap, obvious. Lean runs hot, seizes pistons, drops ring lands — and does it while feeling fast, because lean makes good power right up until it makes scrap. Every jetting decision therefore errs rich when unsure: the manual's stock settings exist to be safe across conditions, and the brave "lean it out for more power" moves belong to people with spare top-ends and dynos. Families new to two-strokes: stock jets, stock needle, stock settings, and change nothing until you can read the evidence.
Reading the plug
The plug chop is the evidence. Procedure, done safely at the track: a clean or fresh plug, a full-throttle run down the longest straight, kill the engine at speed (do not idle down — the idle circuit lies on the plug you are reading), pull the plug in the paddock. Read the insulator around the base of the electrode: cardboard-brown to tan is healthy; white or blistered is lean — stop, go richer; wet, black, sooty is rich — drop down a step. The plug does not lie, and it does not care about your cousin's opinion.
Manual first, folklore never
The starting point is always the manual's chart — the baseline jets, the needle position, the altitude and temperature corrections the manufacturer published. Paddock wisdom is occasionally brilliant, but it is calibrated to someone else's bike, pipe, fuel, and weather; the chart is calibrated to yours. Change one thing at a time, write down what you did (a jetting notebook is a tuner's autobiography), and let the plug confirm each move. Jetting basics in one line: start stock, read the plug, err rich, and remember that the cheapest engine rebuild is the one you never need.