🔧 Jetting Finder
Select your carburetor, build level, altitude, and temperature to get a recommended starting jet.
▸ RECOMMENDED JETTING
Main Jet Size Reference Chart
| Carburetor | Build Level | Sea Level | 2,000 ft | 4,000 ft | 6,000 ft | 8,000 ft |
|---|---|---|---|---|---|---|
| OEM 26mm | Stock | 126–130 | 122–126 | 118–122 | 114–118 | 110–114 |
| Mild | 132–136 | 128–132 | 124–128 | 120–124 | 116–120 | |
| Stage 1 | 140–144 | 136–140 | 132–136 | 128–132 | 124–128 | |
| Stage 2+ | 146–150 | 142–146 | 138–142 | 134–138 | 130–134 | |
| PZ27 (27mm) | Mild | 136–140 | 132–136 | 128–132 | 124–128 | 120–124 |
| Stage 1 | 144–148 | 140–144 | 136–140 | 132–136 | 128–132 | |
| Stage 2+ | 154–158 | 150–154 | 146–150 | 142–146 | 138–142 | |
| PZ30 (30mm) | Stage 1 | 156–160 | 152–156 | 148–152 | 144–148 | 140–144 |
| Stage 2+ | 170–174 | 166–170 | 162–166 | 158–162 | 154–158 |
Needle, Pilot & Air-Fuel Screw Settings
| Carburetor | Build Level | Pilot Jet | Needle Clip | A/F Screw (turns out) | Notes |
|---|---|---|---|---|---|
| OEM 26mm | Stock | #42 | Pos 3 (mid) | 1.5 – 2.0 | Verify pilot jet — leave stock unless idle is rough |
| OEM 26mm | Mild | #45 | Pos 3 – 4 | 1.5 – 2.5 | Re-jet after adding filter + exhaust; lean at WOT otherwise |
| OEM 26mm | Stage 1 | #48 | Pos 3 – 4 | 2.0 – 2.5 | Ensure billet flywheel + rod installed before high-RPM runs |
| OEM 26mm | Stage 2+ | #48 – #52 | Pos 4 – 5 | 2.0 – 3.0 | OEM carb limits Stage 2+ — upgrade to PZ27 or PZ30 |
| PZ27 | Mild | #45 – #48 | Clip 3 (mid) | 1.5 – 2.5 | Good airflow upgrade; may need intake manifold adapter |
| PZ27 | Stage 1 | #48 – #50 | Clip 3 – 4 | 2.0 – 2.5 | Best value carb for governor-removed 301 builds |
| PZ27 | Stage 2+ | #50 – #54 | Clip 4 – 5 | 2.0 – 3.0 | Full tune-up required; multiple plug chops recommended |
| PZ30 | Stage 1 | #50 – #52 | Clip 3 | 2.0 – 2.5 | Use PZ30 only with ported head; overkill on mild builds |
| PZ30 | Stage 2+ | #52 – #56 | Clip 3 – 4 | 2.0 – 3.0 | Full build only — pairs with cam, ported head, forged piston |
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 | –3 to –4 sizes | ~97% | Minor correction; most skip this band |
| 2,001 – 4,000 ft | –5 to –8 sizes | ~93% | Noticeably richer — correction required |
| 4,001 – 6,000 ft | –8 to –12 sizes | ~87% | Significant correction; re-tune A/F screw too |
| 6,001 – 8,000 ft | –12 to –16 sizes | ~81% | Full retune; consider needle clip adjustment too |
| > 8,000 ft | –16 to –20+ sizes | <78% | Full retune recommended; stock power drops 20%+ |
Temperature also affects mixture: add ~2–3 jet sizes for cold weather (<45°F), subtract 2–3 for heat (>90°F).
Carburetor Quick Guide
OEM 26mm Carb
Stock & Mild Builds
- Bore Size ~26mm
- Stock Main Jet #128
- Pilot Jet #42
- A/F Screw 1.5 turns out
- Best For Stock, Mild builds
- Note Larger bore than 212/224 OEM carb
PZ27 (27mm)
Best Stage 1–2 Upgrade
- Bore Size 27mm
- Typical Main Jet #136–#158
- Pilot Jet #45–#54
- A/F Screw 1.5–2.5 turns
- Best For Stage 1–2 builds
- Note May need intake adapter
PZ30 (30mm)
Full-Send Stage 2+ Builds
- Bore Size 30mm
- Typical Main Jet #156–#174
- Pilot Jet #50–#56
- A/F Screw 2.0–3.0 turns
- Best For Stage 2+, ported head
- Note Overkill for mild builds
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 / valve damage 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 loading up or blubbering 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
Most reliable jetting check: run the engine hard at WOT for 10–15 seconds, instantly cut the ignition, and coast to a stop. Pull the plug — tan/light brown = correct, white = lean, black = rich. Always read it before the engine cools.
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 also affects tune. Hot weather (90°F+) requires going down 2–3 sizes. Re-jet when seasons change significantly.
Ethanol Fuel Factor
E10 gasoline (10% ethanol) runs 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.
301 Parts ≠ 212/224 Parts
The 301 is a different displacement class entirely — not a bigger 212 or 224. It runs a larger carb bore and a different fuel curve, so jet numbers from those charts don't carry over. Always use a 301-specific reference.
Frequently Asked Questions
What is the stock main jet size on a Predator 301cc?
The Predator 301cc typically ships with a stock main jet of approximately #128 on the OEM 26mm carburetor. Always remove and read the number stamped on your specific jet before tuning, as production runs and carb brands can vary slightly.
Does altitude affect jetting on the Predator 301cc?
Yes. Thinner air at altitude makes the mixture effectively richer, so you need a smaller (leaner) jet at higher elevations. Drop approximately 3 to 4 jet sizes per 2,000 feet of elevation above your sea-level baseline.
Can I use a Predator 224 or 212 jetting chart for the 301cc?
No. The 301cc has significantly more displacement and requires a larger carburetor bore than the 212 or 224. The absolute jet numbers are much higher, and the fuel curve behaves differently. Always use a 301-specific reference.
Related Calculators
224 Jetting Chart
The 224's jetting reference — different absolute numbers, same tuning principles.
212 Jetting Chart
The original 212 jetting reference for smaller-bore builds.
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