Rotax Max carburettor jetting: the EGT method
A Rotax Max is jetted on exhaust gas temperature, not on feel. Here is the full decision logic: which part controls which load range, and which EGT value triggers which change.
1. What each part does
| Part | Range | Effect |
|---|---|---|
| Main jet | Full throttle | Bigger size = richer, EGT drops. Smaller = leaner, EGT rises. |
| Needle (clip) | Mid range | Raising the clip lowers the needle and leans mid range. Lowering the clip richens it (corner exits). |
| Air screw | Low range | Opening it (unscrewing) leans the low range. Closing it (screwing in) richens it. |
| Needle jet / emulsion tube | Mid and full throttle | Number fixed by homologation, but manufacturing tolerances change the clearance around the needle: a wider bore richens, a tighter one leans. |
| Pilot jet | Idle | Not changeable in competition. |
2. The reference EGT window on a Rotax Max
The target working range on a Rotax Max senior is roughly 370 °C representative minimum and 680 °C representative peak. Read these as percentiles over the session, not as the raw maximum: an isolated 720 °C sample means nothing — what matters is how the high level holds.
3. The decision tree, in order
Step 1 — Set full throttle (main jet)
- Peak above 740 °C: clearly too lean. Increase the main jet immediately — real engine risk.
- Peak between 710 and 740 °C: slightly lean. Main jet +1.
- Peak between 650 and 710 °C: in the window. Leave it alone.
- Peak between 600 and 650 °C: slightly rich. Main jet −1 if you are chasing performance.
- Peak below 600 °C: clearly too rich. Main jet −2, the engine will not pull.
Step 2 — Set low and mid range, only once the peak is right
- Minimum EGT below 320 °C: too rich at low load. Lean one step with the needle (raise the clip) or open the air screw slightly.
- Minimum EGT above 460 °C: too lean at low load. Richen the needle (lower the clip).
- Between the two: jetting validated.
Step 3 — Confirm with the end-of-straight trend
- EGT drops by more than 15 °C at the end of the straight: too rich at high revs, main jet −1 to −2.
- EGT keeps climbing to the end of the straight without stabilising: too lean, main jet +.
4. The track changes the setting, not just the weather
The main jet mostly follows air density: temperature, atmospheric pressure, altitude, humidity. That is a weather calculation.
The needle follows the track. With identical weather, two circuits can call for two different clip positions:
- Fast track, long straights: a lot of full-throttle time, the needle does little work, the main jet dominates.
- Slow, technical track: many part-throttle acceleration phases out of corners, so needle and air screw become decisive.
- High grip: better loading on corner exit, therefore a richer needle requirement.
5. Reading EGT by track zone
| Zone | Indicative EGT | Part involved |
|---|---|---|
| Slow corner exit | 490 – 540 °C | Air screw |
| Part throttle, sequences | 580 – 640 °C | Needle (clip) |
| End of straight | 610 – 650 °C | Main jet |
6. Guard rails
- One change at a time, otherwise you cannot tell what worked.
- If the needle is already at an extreme position, the clip is no longer the answer: go through the main jet or air screw.
- Repeated misfires (more than three per lap) invalidate the reading: sort ignition and spark plug first.
- An EGT rise above 100 °C per second is an alarm, not a setting: stop and inspect the engine.
KartAnalytics computes these percentiles automatically from session telemetry, locates the zones on the track map and suggests the matching setting.
Frequently asked questions
What EGT should I target on a Rotax Max?
Roughly a window of 370 °C representative minimum and 680 °C representative peak, read as percentiles over the session rather than as the raw maximum.
Does raising the needle clip richen or lean?
Raising the clip lowers the needle into the needle jet and leans the mid range. Lowering the clip raises the needle and richens it.
What does opening the air screw do?
Unscrewing the air screw lets more air in and leans the low range, typically at slow corner exits and idle. Screwing it in richens.
Can I change the pilot jet or the needle jet?
No. On a homologated Rotax Max both are fixed. Only manufacturing tolerances on the needle jet slightly change mixture strength.
Should jetting change between two tracks in identical weather?
Rarely the main jet, which follows air density. Needle position often does change, because part-throttle time depends on the track layout.
Analyse your telemetry in seconds
Import your file, compare laps, get setup advice.
Create an account