Carburetor Tuning 101: Adjusting High & Low Speed Screws 🔧

So, you’ve cleaned your air filter, installed fresh fuel, and put in a new spark plug—but your engine still runs rough. If it struggles to accelerate or idles poorly, you might need to adjust the carburetor mixture screws.

This guide is for small engines that feature adjustable fuel/air mixture screws (often found on two-stroke equipment like chainsaws, trimmers, and some older lawnmowers). Caution: Improper tuning can quickly lead to engine damage. Always prioritize your engine’s health over maximum power.

Understanding the Three Screws

Adjustable small engine carburetors typically have three external screws to control the fuel/air mixture and speed:

The goal of tuning is to find the sweet spot where the engine runs as fast and smoothly as possible without overheating (running too lean).

The Crucial Starting Point: Base Settings

Before you start turning screws, you must establish the manufacturer’s base setting. This is the safest place to start.

1. Gently seat the H and L screws: Turn both the High (H) and Low (L) screws clockwise until they gently stop. DO NOT OVERTIGHTEN! This can damage the needle and its seat, ruining the carburetor.
2. Back Out to Base: Turn the screws back out (counter-clockwise) to the manufacturer’s recommended base setting. A common starting point for many two-stroke engines is 1 to 1.5 turns out on both the H and L screws, but always check your specific manual first.

Step-by-Step Tuning Procedure

Always perform adjustments with the engine fully warmed up (2-3 minutes of running).

1. Tune the Low Speed (L) Screw

The Low Speed screw ensures a good idle and smooth transition to mid-speed.

• Warm Up & Idle: Start the engine and let it idle. If it dies, slightly adjust the Idle Speed (T) screw clockwise until it stays running.
• The “Peak RPM” Method: Slowly turn the L screw clockwise (leaner) until the engine RPM starts to drop or the engine begins to stumble. Then, slowly turn the screw counter-clockwise (richer) until the engine RPM starts to drop again.
• Set the Middle: Set the L screw exactly in the middle of those two points where the engine ran its fastest and smoothest.
• Final Idle Check: Adjust the Idle Speed (T) screw back down so the engine idles without the cutting attachment (chain, trimmer head) moving.

2. Tune the High Speed (H) Screw

The High Speed screw is the most critical for engine protection and power.

• Full Throttle: Hold the engine at Wide Open Throttle (WOT).
• Find the Peak: Slowly turn the H screw clockwise (leaner) while listening to the RPM. The engine sound will change from a sputtering “four-stroke” sound to a clean, crisp “two-stroke” sound as the speed increases. Stop turning when you hear the maximum RPM.
• Enrich for Safety (The “1/4 Turn Rule”): Once you find the maximum RPM, turn the H screw back out (counter-clockwise) 1/8 to 1/4 of a full turn. This adds a crucial bit of extra fuel, slightly lowering the top RPM but enriching the mixture. This extra fuel is essential for cooling the engine when it is under load. Running the engine too lean (at its absolute highest RPM setting) under a working load will cause high heat and almost certainly lead to piston or cylinder damage.

Troubleshooting Symptoms

Carburetor tuning is more of an art than a science, requiring a patient ear and a respect for the power of the fuel mixture. Use the base settings, follow the safety margin on the High Speed screw, and your small engine will deliver reliable, smooth power for years to come.