How to Tune a 2-Stroke Carb

By David Banks ; Updated April 12, 2017
Classic mopeds have easy-to-tune 2-stroke carbs.

Two-stroke carburetors are perfect practice for novice mechanics. They are easy to work with and do not require excessive motor knowledge to achieve desired engine performance. Two-stroke carbs are usually found on leaf blowers, motorized bikes and mopeds. Improper tuning of your carb will definitely lead to lagging performance and it might also lead to long-term damage to your motor. Proper tuning can increase the performance and longevity of your engine.

Locate the RPM jets on your carburetor. Some carbs have a high jet (marked with an H) and a low jet (marked with an L). Others have only one jet. These jets adjust the carburetor's performance at different RPMs.

Turn the jets completely clockwise with your flat-head screwdriver to completely shut them.

Turn the low jet counterclockwise 1 and 1/4 turns. This is easy to do with a screwdriver. Just count the rotations you make, being sure to stop a quarter past your first complete rotation.

Turn the high jet counterclockwise approximately 1 and 3/8 turns. These are the basic starting positions for carb jets. Depending on whether your engine is running lean (too much air and too little gas) or rich (too much gas and too little air) you will adjust your carb jets to tune your engine.

Start your engine. If it does not start, turn your low jet slightly back and forth until the engine starts. Let your engine run for a minute. If it dies, your carbs are running too lean. Close the low jet (turn clockwise) in 1/16th turn increments until the motor continues to run. If it is idling too high, open the jet (turn counter-clockwise) in similar increments. The low jet is tuned when the engine idles smoothly without revving or sputtering.

Rev your engine full-throttle to tune your high jet. Close the jet (turn clockwise) in 1/16th increments until you reach you maximum high-end RPM. After you have found your maximum RPM, open the jet (turn counterclockwise) 1/16th of a turn. This will ensure the engine is not running too lean and protect your engine from damage.