🔧 Jetting Finder

Select your setup to get a recommended main jet size.

Carburetor

Build Level

Altitude

Ambient Temp

▸ RECOMMENDED JETTING

Main Jet Size Reference Chart

Carburetor	Build Level	Sea Level	2,000 ft	4,000 ft	6,000 ft	8,000 ft
22mm Clone	Stock	98–100	95–97	92–95	89–92	86–89
	Mild	100–104	97–101	94–98	91–95	88–92
	Stage 1	105–110	102–107	99–104	96–101	93–98
	Stage 2+	110–118	107–115	104–112	101–109	98–106
Mikuni VM22	Stock	100–105	97–102	94–99	91–96	88–93
	Mild	105–108	102–105	99–102	96–99	93–96
	Stage 1	108–115	105–112	102–109	99–106	96–103
	Stage 2+	115–125	112–122	109–119	106–116	103–113
PZ27	Mild	100–105	97–102	94–99	91–96	88–93
	Stage 1	105–112	102–109	99–106	96–103	93–100
	Stage 2+	112–122	109–119	106–116	103–113	100–110
PZ30	Stage 1	108–115	105–112	102–109	99–106	96–103
PZ30	Stage 2+	115–130	112–127	109–124	106–121	103–118

Needle, Pilot & Air-Fuel Screw Settings

Carburetor	Build Level	Pilot Jet	Needle Clip	A/F Screw (turns out)	Notes
22mm Clone	Stock	#42	Pos 3 (mid)	1.5 – 2.0	Factory default — decent baseline
22mm Clone	Mild	#42	Pos 3 – 4	1.5 – 2.5	Move clip to Pos 4 if midrange is lean
22mm Clone	Stage 1	#45	Pos 3 – 4	2.0 – 2.5	Upjet pilot if idle/transition is lean
22mm Clone	Stage 2+	#45 – #48	Pos 4 – 5	2.0 – 3.0	Full tune-up required; plug chops recommended
Mikuni VM22	Stock	#17.5 – #20	Clip 3 (mid)	1.5 – 2.0	Pilot is a different standard from clone carb
Mikuni VM22	Stage 1	#20 – #22.5	Clip 3 – 4	2.0 – 2.5	Use OEM Mikuni needle (6DH1) for best atomization
Mikuni VM22	Stage 2+	#22.5 – #25	Clip 4 – 5	2.0 – 3.0	Consider aftermarket needle for aggressive builds
PZ27	Mild	#38 – #42	Clip 3 (mid)	1.5 – 2.5	Budget option; quality varies significantly
PZ30	Stage 2+	#40 – #45	Clip 3 – 4	2.0 – 3.0	Only use PZ30 with ported head & large-bore kit

Altitude Correction Quick Reference

Thinner air at altitude = leaner mixture. Drop jet size by approximately the amount shown below from your sea-level baseline.

Altitude Band	Approx. Jet Correction	Air Density vs. Sea Level	Tip
Sea Level – 1,000 ft	Baseline (no change)	100%	Sea-level jet is your reference point
1,001 – 2,000 ft	–2 to –3 sizes	~97%	Barely noticeable; most skip this correction
2,001 – 4,000 ft	–4 to –6 sizes	~93%	Noticeably richer at altitude — correction required
4,001 – 6,000 ft	–6 to –9 sizes	~87%	Significant correction needed; re-tune A/F screw too
6,001 – 8,000 ft	–9 to –12 sizes	~81%	Full retune recommended; consider needle clip too
> 8,000 ft	–12 to –16+ sizes	<78%	Consult a dyno; stock power drops ~20%+

Temperature also affects mixture: add ~2–3 jet sizes for very cold weather (<45°F), subtract 2–3 for heat (>90°F).

Carburetor Quick Guide

22mm Clone Carb

Stock & Budget Builds

Bore Size 22mm
Stock Main Jet #98–100
Pilot Jet #42
A/F Screw 1.5 turns out
Best For Stock, Mild mods

Mikuni VM22

Best All-Rounder Upgrade

Bore Size 22mm
Typical Main Jet #100–115
Pilot Jet #17.5–22.5
Needle 6DH1 (stock)
Best For Stage 1–2 builds

PZ27

Budget Performance Step-Up

Bore Size 27mm
Typical Main Jet #100–120
Pilot Jet #38–45
A/F Screw 2.0–2.5 turns
Best For Mild–Stage 1

PZ30

Full-Send Stage 2+ Builds

Bore Size 30mm
Typical Main Jet #108–130
Pilot Jet #40–48
A/F Screw 2.0–3.0 turns
Best For Stage 2+, ported head

Lean vs. Rich Symptoms

🔴 RUNNING LEAN (jet too small)

Engine overheats quickly
Flat or hesitant acceleration
Popping / backfire on deceleration
Engine surges at steady throttle
White or light grey spark plug tip
Loss of power, especially at WOT
Piston damage / seizure risk if ignored

FIX → Install a larger main jet (go up 2–4 sizes and retest)

🔵 RUNNING RICH (jet too large)

Black smoke from exhaust
Wet, black, or sooty spark plug
Sluggish low-end, poor throttle response
Fuel smell at exhaust
Fouled spark plug; hard to restart
Engine blubbering / loading up at WOT
Increased fuel consumption

FIX → Install a smaller main jet (go down 2–4 sizes and retest)

Jetting Tips & Best Practices

🔌

Plug Chop Method

The most reliable way to check jetting: run the engine hard to WOT for 10–15 seconds, then instantly cut the ignition and coast to a stop. Pull the plug — tan/light brown = correct, white = lean, black = rich.

📏

One Change at a Time

Never change main jet, pilot, and needle all at once. Start with the main jet for WOT behavior. Then dial in the needle clip for midrange. Lastly, adjust the pilot/A-F screw for idle and off-idle response.

🌡️

Temperature & Humidity

Cold dense air acts like lower altitude — go up 2–3 jet sizes below 45°F. High humidity can also affect tune. Hot weather (90°F+) requires going down 2–3 sizes. Re-jet when seasons change significantly.

⛽

Ethanol Fuel Factor

E10 gasoline (10% ethanol) is leaner than pure gasoline. If switching to non-ethanol or E0 pump gas, you may need to go up 2–3 jet sizes. Always note your fuel type when recording a tune.

🔑

A/F Screw Baseline

Gently seat the A/F screw (don't force it) then back out 1.5 turns as a starting point. Adjust in 1/4-turn increments at idle — out = richer at idle, in = leaner. Target the highest, smoothest idle speed.

🏁

Race Day Tuning

Check weather forecasts before a race. If temperature drops 20°F from your baseline tune, go up 2 jet sizes. If you're racing at a higher-altitude track than where you tuned, bring spare leaner jets.

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