Intake Air Temp
Diagnostics

This tool rarely fixes anything by itself. What it does is stop you from fixing the wrong thing.

Start with the idea, because everything on this page hangs off it. A carburetor delivers roughly the same fuel for a given throttle opening, no matter what the air is doing. But hot air is thin air: fewer oxygen molecules in every gulp. So on a hot day, the same jetting runs effectively richer. Cold air is dense air, packed with oxygen, and the same jetting runs effectively leaner. The page does that math for you: air at a hundred and ten degrees carries about seven percent less oxygen than air at seventy. Your jetting feels every bit of that seven percent.

Now use it before you touch a jet. The bike runs a little fat on a hot afternoon, or a little lean on a cold morning, and the instinct is to reach for the jetting box. Read this page first. If the intake number explains the change, the weather did it, the jetting is innocent, and the right move is to leave the carburetor alone. Jets are for changes that stay changed; weather passes.

Watch for the trap this tool catches better than any other: the hot restart. Park a hot bike for ten minutes and the intake tract heat-soaks. For the first minutes after you restart, the air entering the carb can run thirty or forty degrees above the day's temperature, and the bike runs rich and lazy like its jetting went bad sitting still. It did not. Ride it, let moving air flush the heat out, watch the number fall back toward the day, and judge nothing until it does.

Log the condition with the session, the same way you log head temp: intake temperature and the day's ambient, written next to the tune. A jetting card that knows its weather can be trusted in any season. One that does not will have you chasing your own tail every time the season turns.

A carburetor is a faithful but simple servant: for a given throttle opening, it delivers roughly the same fuel, every time, whatever the air is doing. The air, though, is never doing the same thing twice. Hot air is thin air, with fewer oxygen molecules in every gulp the engine takes, so the same fuel charge runs effectively richer. Cold air is dense air, packed with oxygen, and the same jetting runs effectively leaner. The math runs on absolute temperature, and it is not small: air at a hundred and ten degrees carries about seven percent less oxygen than air at seventy. Stacked on top of the weather is the engine itself: a hot intake tract heat-soaks, and at idle, and for minutes after a restart, the air entering the carb can run thirty or forty degrees above the day outside. This diagnostic reads a temperature sensor mounted in the intake tract, ahead of the carburetor, and separates those two pushes, so you can see which way conditions are leaning before you blame the jetting.

The top of the page is the live reading: the intake air temperature itself, beside the day's ambient temperature, so the heat-soak gap between them is a number and not a feeling. The page turns the reading into its consequence: how much oxygen this air is carrying compared with your reference temperature, shown as a percentage, and which way that pushes your mixture, richer or leaner, and by roughly how much. That reference temperature is yours to set; it is the condition you jetted the bike in, and every reading on the page is judged against it. Your settings here are saved on this device.

The controls below let you rehearse the year. The ambient control moves the day's temperature, from a cold morning start to a hot afternoon, and the reading answers the way the real sensor will. The airflow control is the other half of real life: standing still in traffic, the intake bakes and the number climbs away from ambient; rolling down the road, moving air flushes the heat out and pulls it back toward the day. A temperature-unit switch flips the page between Fahrenheit and Celsius, whichever your manual and your habits speak.

The lesson sections at the bottom make the two habits this tool exists to build. First: check this page before you re-jet for the weather. A bike that runs fat on a hot afternoon and lean on a cold morning is often a correctly jetted bike in moving air, and jets are for changes that stay changed. Second: beware the hot restart. Judge nothing in the first minutes after restarting a hot bike, while the intake is still soaked and the reading is lying about the day. Log the intake temperature beside your tuning sessions, next to the head temp, and a jetting card that knows its weather can be trusted in any season.