Cylinder Head
Temp Diagnostics

Start the engine cold and just watch the climb. The number rises fast at first, then the climb flattens as the head soaks itself through with heat. Do not tune anything yet. A carbureted engine is a moving target while it warms: fuel does not atomize the same cold, and an idle set at a hundred and fifty degrees will be wrong at three hundred and forty. Cold readings are not imprecise; they are measuring a different engine.

The gate at the bottom of the page runs the session. Three rows have to check at the same time. The head is inside the operating band, three hundred to four hundred degrees at the plug. It is holding steady, moving less than a couple of degrees at a time instead of still climbing. And it has soaked there, stable in the band for a full minute, so the head, the intake, and the carb body are all at temperature together. When all three rows check, the lamp lights. That lamp is the tool refusing to let you tune a cold engine, and it is right.

Now tune, and tune in one sitting: mixture, sync, TPS, jetting checks, whatever is on the bench today. Every adjustment you make now is comparable, because the temperature is not moving underneath it. Keep one eye on the top of the band during long parked sessions, because idling in still air stacks heat. If it pushes past four hundred, take a short easy ride, shed the heat, let it settle back into the band, and pick up where you left off.

When the session ends, remember the number. A tuning note that says set at three hundred and forty-five degrees head temp on a seventy-degree day can be reproduced next month. One that just says Tuesday cannot. The temperature is part of the tune; this page is how you know it.

Everything you will ever tune on a carbureted engine is only true at one temperature: fully warm, and holding still. While an engine warms up, it is a moving target. Fuel does not atomize the same cold, the enrichener circuit is still in play, and every setting you take is drifting underneath you as the metal soaks up heat. So before mixture, sync, TPS, or jetting mean anything, somebody has to answer one question honestly: is this engine at operating temperature, and has it settled there? This gauge is that answer. It reads from a ring thermocouple sandwiched under the spark plug, the hottest honest point on the head, the same place plug readings have always been judged from.

The top of the page is the gauge itself: the head temperature, big, with the operating range drawn under it as a bar running cold, warming, operating, too hot. Normal for an air-cooled twin at the plug is three hundred to four hundred degrees; past four hundred twenty-five, you are in overheat country. Beside the number, the page shows the day's ambient temperature, because the head starts at ambient and the day is part of every reading, and a verdict box tells you plainly whether you are ready to tune or not, and why.

Below the gauge is the load control. Load is heat: more throttle makes more heat than the fins can shed standing still, and here you can walk the load up and watch the needle answer the way the real engine would. Then the Conditions section, in cards. Ambient temperature, because a forty-degree morning warms up nothing like a ninety-five-degree afternoon. Airflow over the fins, because moving air is the entire cooling system an air-cooled head has, and a tune set parked-hot can sit somewhere else rolling down the road. Warm-up speed, a demonstration control that speeds the clock so you can study the shape of the climb instead of waiting on it. And a sensor card that reminds you which reading you are looking at: one sensor per head is possible, and on a V-twin the rear cylinder usually runs hotter than the front.

The warm-up trace draws the session: temperature against time, against the operating band. Learn that shape: a fast early climb that flattens as the head soaks. Tuning starts where the line goes flat inside the band, not before.

Then the page's conscience, the ready-to-tune gate. Three rows: in the operating band; holding steady, moving less than two degrees in ten seconds; and soaked, a full minute stable inside the band. The lamp lights only when all three check at once. Kill the engine, and the gate closes, because a cooling head is a moving target again.

Below, the page teaches its numbers in a reading table and lays out how a session goes, step by step, including logging the temperature with the tune. The Ride Monitor and Trends sections are Premium's layer: the monitor watches the gauge for a whole ride and calls your phone the moment a threshold is crossed, and Trends watches your peak cruise temperature across sessions, so a head creeping hotter week over week, from a drifting mixture or fins packing with grime, gets flagged while it is still a small fix.