Driver Portal
Carburetor Jetting
Physics-based needle and jet recommendations based on altitude, temperature and air density.
Engine & Carburetor
BAR
0.76
Weather & Conditions
Needle Settings — Tillotson HW-27A
H — High Needle
Left side · Air intake · WOT mixture
—
— h : — m
Turn counter-clockwise ↺ to set
L — Low Needle
Right side · Engine · Low/mid throttle
—
— h : — m
Turn counter-clockwise ↺ to set
Pop-off
—
BAR
Exhaust
—
mm
Air Density
—
lb/ft³
Density Alt.
—
m
Fine Tuning (±¼ turn)
+0.0
+0.0
Adjust after plug chop. Each step = ¼ turn. Richen = positive / Lean = negative.
Spark Plug Guide
Where to read
🔍
Insulator nose
The ceramic cone around the centre electrode. This is your primary colour reference — always read here first.
⚡
Centre electrode
Look for erosion, rounding, or melting. A sharp square edge = healthy. Rounded or pitted = high heat or lean running.
🪝
Ground electrode
Check for colour match with insulator. Blueing or burning on the ground electrode alone indicates detonation.
Colour readings
Mid-brown / biscuit / tan
Insulator nose is a warm earthy brown. Slight grey ring at the base.
✅ Perfect — no change needed
Dark grey / light soot
Grey-brown tint, slightly darker than ideal. Common on cool mornings.
Slightly rich → lean H ⅛ turn in
Black / sooty / wet
Heavy carbon, possibly wet or oily to the touch. Fouled plug territory.
Too rich → lean H ¼–½ turn in
Light grey / chalk white
Insulator looks pale, almost clean. Electrode tip may show slight greyish tinge.
⚠️ Lean → richen H ¼ turn out
Bright white / glazed
Porcelain-clean, sometimes with a glassy sheen. Insulator almost looks new.
🚨 Very lean → richen H ½ turn out — recheck
Blistered / melted electrode
Ceramic cracked or blistered. Electrode tip rounded, pitted, or partially melted.
🚨 STOP — engine damage risk. Do not restart.
Other signs to watch
Oil fouling
Wet, oily black coating — not just carbon. Often indicates worn piston rings, too much oil in mix, or a flooded start. Check oil ratio first.
Detonation
Small black pepper-like specks on insulator, or tiny metal fragments from the piston crown fused to the ceramic. Caused by ignition timing, very lean mixture, or bad fuel. Stop and investigate.
Pre-ignition
Melted or eroded ground electrode with a white or clean insulator. Heat range may be too hot, or ignition timing is off. Replace plug and check heat range spec for your engine.
How to do a plug chop
1
Install a fresh, cold plug — never read a used plug, it carries old deposits.
2
Run 3–4 full-speed laps at race pace — including full-throttle sections and hard braking.
3
Kill the engine at full throttle on the straight entering pit — do not idle, do not coast. Kill switch immediately.
4
Remove plug and read the insulator nose within 30 seconds — colour fades quickly as it cools.
5
Adjust jetting, repeat with another fresh plug — never re-read the same plug after adjusting.