Twelve Laps of Fuel and a Feature at Twenty Past

Sit. Don't sit on the jets. Okay — a main jet is a hole, and the number stamped on it is how wide the hole is in thousandths of an inch. Sixty-four is sixty-four thousandths. Flow through a hole goes as the square of the diameter. So sixty-four to sixty-eight is not four more of anything, it's — sixty-eight squared over sixty-four squared, that's 4624 over 4096, that's about thirteen percent more fuel. Thirteen. Say it back to me.

That's why everybody steps two numbers. Two numbers is six and a half percent. Four numbers is thirteen and if you're jumping four numbers you're not tuning, you're guessing, and then you're reading a plug that's telling you the truth about a guess.

And the can — somebody borrowed this can in June and put four jets back in it without turning them over to read the stamp. There are four sixty-sixes in here. Four. I have one sixty-six and three liars and I can't tell you which is which.

Three circuits, and fixing one does nothing to the others. Idle comes off the transfer slot and those two mixture screws. Cruise is the main jet, through the emulsion tube. Full throttle is the main jet plus the power valve, which is a spring and a diaphragm that opens when manifold vacuum drops under its number and dumps in extra. So if it's right everywhere and only goes black when it's pulling hard off two, that's a power valve too big and there is no jet change on earth that fixes it without wrecking your cruise.

Float level is upstream of every bit of it. The float sets how high the fuel stands in the bowl and every jet is drawing off that level. Set it an eighth high and you've richened the idle circuit, the main circuit and the power circuit all at once, and now every jet in this can reads two numbers bigger than it is. That's why you check float first. Nobody checks float first.

Air density. The motor swallows a volume and burns a mass. Cold, dry, high barometer — that's more oxygen through the same hole and it wants more fuel. Hot, muggy, low barometer — less. Fifty-five degrees this morning to ninety-one right now is two jet sizes, easy, and everybody who didn't change is rich now or was lean then. And humidity runs backwards from how you'd figure: water vapor is lighter than air, so a damp afternoon is pushing oxygen out of the way, and damp wants less fuel. Half the pit has that upside down. The track got watered at three-thirty, so it's coming up damp in here right now while we sit.

Air bleeds. Nobody touches them and they're the fine adjustment. They emulsify the fuel up in the main well. A bigger high-speed bleed leans the top end without leaning the middle — it changes the shape of the curve, not the height of it. About a third of what people throw jets at is a bleed.

The bowl sloshes. Left turn only, all night. Fuel piles up against the outboard wall and the inboard jet comes uncovered for a half second in the middle of the corner. Good on the straight, hiccups halfway through three and four — that's fuel moving, not jetting, and every new person in this pit changes jets for it and makes it worse.

Lean doesn't just go slow. Lean goes hot and puts a hole in a piston. Rich washes the oil off the cylinder wall and eats a ring set in one night. Both expensive, and not the same expensive. We have not holed a piston since October of 1984 and I intend to keep saying that.

Jet four in there, the stamp's gone — somebody took pliers to it and wiped the number smooth. I've thought for two years it's a seventy, because it fell out of the drawer with the seventies and I had nothing else in that drawer. Only way to know is run it and read it.