You don't adjust a jet, you swap it for another hole
A jet size is a hole measured in hundredths of a millimetre, and there's no dial for it. Tuning a carburettor means unbolting one and choosing a different hole from a row.
Five small brass cylinders, drawn in section, each stamped with a number: 105, 112, 120, 128, 135. Beneath them, an arrow and four words — RICHER THIS WAY.
There's no dial anywhere on this design. That absence is the entire subject.
What is a jet number actually measuring?
The diameter of a hole, in hundredths of a millimetre.
A 105 jet has a bore of 1.05mm. A 135 has a bore of 1.35mm. That's the whole instrument — a precisely drilled restriction that fuel is pulled through by the engine's own airflow, and the size of the hole decides how much fuel gets through for a given amount of air. Nothing else about the jet varies. It's a number stamped on a part whose entire function is described by that number.
Why can't you just turn something to adjust it?
Because a hole doesn't have a middle setting.
A modern fuel-injected engine adjusts mixture electronically, continuously, thousands of times a second. A carburettor jet is fixed the moment it's drilled — there's no needle to nudge partway between 120 and 128, because 120 and 128 are two different pieces of brass. Getting the mixture right means removing the one that's in there and threading in a different one, which is why they come as a row rather than a range.
Why does richer mean a bigger number?
Because a bigger hole lets more fuel through for the same pull of air.
Richer means more fuel relative to air; leaner means less. The arrow points at 135 because that's the largest hole on this row, and it's labelled rather than assumed, because the direction isn't obvious from the numbers alone until you've had it explained once. After that it never needs explaining again.
What decides which jet is right?
Altitude, temperature, and what the engine's actually been modified to do — none of which a jet can sense for itself.
Thinner air at altitude needs a smaller jet, because there's less oxygen for the same volume of air pulled through. A hotter day needs the same adjustment for the same reason. An engine that's been built to breathe harder than it did when the carburettor was matched to it usually wants more fuel, not less. None of this is guesswork if you know what to test for — it's the same kind of number a spark plug gap card checks against a fresh part: a figure you verify rather than assume.
Getting it wrong in either direction
Too lean and the engine runs hot and can hole a piston under load. Too rich and it fouls plugs, wastes fuel, and coats the inside of the exhaust in unburned residue. Both failures are slow rather than sudden, which is why the row of five exists — nobody gets it right by guessing once.
Does this apply to anything beyond old cars?
Small engines everywhere still use exactly this method.
Motorcycles, generators, chainsaws, outboard motors — any carburetted engine tunes the same way, by swapping a drilled part for a different drilled part rather than turning a dial. It's a smaller, older idea than most people expect from something still in daily use on machinery nobody thinks of as vintage.
Can a real carburettor's part numbers go on the shirt?
The bore sizes are physics, and nobody owns a diameter.
A manufacturer's own part-numbering system for a specific carburettor model is a different matter — that's their catalogue, licensable the way any published document is if a real one is the point. Left generic, the row of five stays exactly what it looks like: a tuning kit, not a catalogue page.
Does it print well?
Very — five small circles in section is about as clean as artwork gets.
Flat brass or gold against black keeps the drawn sections legible without a gradient anywhere. Set the numerals large — they're the whole design, so there's no reason to shrink them to match a real workshop chart, and fine numerals below the weave's floor fill in and stop reading as numbers at all.
How do I get one made?
Describe the set at JustOG — how many jets, the sizes, and which one the arrow should point at. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped.
Designs other people have published are in the shop.
Five holes in a row, and getting it right is a matter of picking the correct one.