Carburetor Tuning Guide
CARBURETOR TUNING GUIDE
Stop guessing at jets. Tune one throttle range at a time and let the engine tell you what it wants.
Pit Bike, CT70 & Small Engine Carburetor Tuning
This guide covers pit bike tuning, Honda CT70 and Z50 tuning, ATC70 tuning, 4-stroke mini bike tuning, and aftermarket horizontal engines including Zongshen, YX, Lifan, and Daytona motors. The same basic process applies to many NIBBI, Keihin-style, Mikuni-style, PE, PZ, VM-style, and other round-slide carburetors.
Don't change the pilot jet, needle, and main jet at the same time. Get one circuit working correctly, then move on to the next.
Know What You're Tuning
Carburetor circuits are tuned by throttle position, not engine RPM or vehicle speed. Use your physical throttle opening to determine which adjustment to work on.
The circuits overlap. The chart shows the throttle ranges where each adjustment has the greatest influence.
Pilot / Slow Jet
The pilot circuit controls most of the fueling from idle through small throttle openings. If the bike has trouble idling, hesitates when you first open the throttle, or bogs below about 1/4 throttle, start here.
Warm the engine completely before making adjustments. The mixture screw will help tell you whether the pilot jet is too large or too small.
Air screws and fuel screws work in opposite directions. Use the diagram below to identify which type your carburetor has, then select it to see the correct tuning instructions.
Think of the pilot jet as the coarse adjustment and the mixture screw as the fine adjustment. If the screw ends up near either extreme, change the pilot jet and tune it again.
Main Jet
Once the low-speed circuit is working correctly, move to the top end. At large throttle openings, the main jet becomes the primary fuel restriction.
Test under load. Revving the engine in neutral won't give you an accurate picture of how the main jet is working.
If the engine bogs at 3/4 to full throttle, apply a small amount of choke/enrichment and see what happens.
Runs better with choke → LEAN
Try a larger main jet.
Runs worse with choke → RICH
Try a smaller main jet.
Change the main jet in small increments and test again. You're looking for clean acceleration and strong power through the upper throttle range.
LEAN: Often feels flat or hollow. The engine may stop pulling cleanly as you open the throttle.
RICH: Often feels heavy or muffled. The engine may sputter, break up, or struggle to rev cleanly.
Needle Position
The needle has its greatest influence through the middle of the throttle range. Get the pilot and main jet close first, then use the needle to clean up the midrange.
Needle adjustments are easy to test. On most carbs, you can remove the slide, move the clip one position, reinstall it, and test again without draining the carburetor or changing jets.
CLIP DOWN ↓
Needle moves up → RICHER
CLIP UP ↑
Needle moves down → LEANER
If you don't have a known-good starting point, begin near the middle clip position. Move the clip only one position at a time.
Hold the throttle in the problem area and apply a small amount of choke/enrichment.
Runs better with choke → LEAN
Raise the needle by moving the clip down one position.
Runs worse with choke → RICH
Lower the needle by moving the clip up one position.
Everything Seems Lean?
If every circuit seems to want considerably more fuel, check the basics before installing larger jets everywhere.
- Fuel flow from the tank and petcock
- Kinked or restricted fuel line
- Float height / fuel level
- Clogged jets or passages
- Intake or manifold air leaks
- Loose carburetor or intake connections
A fuel-delivery problem or air leak can make several throttle ranges appear lean at the same time.
Quick Carburetor Size Guide
Carb size is a separate subject from jetting, but starting with a reasonable size makes tuning much easier. These are good starting points for typical 4-stroke minibike and Honda-style horizontal engines.
| Engine | Carb | Engine | Carb |
| 50cc | 14mm | 125cc | 22mm |
| 70cc | 17mm | 140cc | 24mm |
| 90cc | 19mm | 160cc | 26mm |
| 110cc | 20mm | 190cc | 28mm |
| 212cc | 30mm |
These are starting points, not hard limits. Engine modifications and intended use can change the ideal size. Bigger isn't automatically better.
Carb Tuning Cheat Sheet
| Throttle | Adjustment | Test |
| Idle–1/4 | Pilot | Mixture screw |
| 1/4–3/4 | Needle | Clip / choke |
| 3/4–Full | Main | Choke |
TEST IT. REPEAT.
Work through each circuit individually and let the engine's response tell you which direction to go. Small changes and consistent testing will get you there faster than changing several things at once.