How to Tune a Scooter Carburator
By: Peter Dahlin
Tags: Carburetor, Scooters, Cars, Moped, Racing

This guide to tuning a carburetor will explain how to correctly adjust the gas/air mixture. A carburetor that is tuned correctly will both protect your engine and induce optimal performance. If you are interested in more scooter information, forums, and scooter t-shirts, hoodies, and more, visit scootzr.com.

Terminology

Lean: referring to the tuning of a carburetor in which the fuel/air mixture contains too little fuel and too much air

referring to the tuning of a carburetor in which the fuel/air mixture contains too little fuel and too much air Rich: referring to the tuning of a carburetor in which the fuel/air mixture contains too much fuel and too little air

referring to the tuning of a carburetor in which the fuel/air mixture contains too much fuel and too little air Needle: a needle shaped screw which manipulates the mixture ratio that the carburetor creates

a needle shaped screw which manipulates the mixture ratio that the carburetor creates High Needle: the needle that adjusts what the mixture ratio will be athigh speeds

the needle that adjusts what the mixture ratio will be athigh speeds Low Needle: the needle that adjusts what the mixture ratio will be at low speeds (some carburetors do not have an adjustable low speed needle)

The mixture’s effects

A lean condition will run hotter because the lean mixture burns at a higher temperature and because less lubrication is making it into the engine. A two stroke engine gets it oil lubrication from the gas and if there is less gas in the mixture, that means that less oil is making it to the engine.

There is a difference between the best tune and the highest performance tune. When tuned to the level of the highest performance, a two stroke engine will be running in a slightly lean(too much air and too little gas) condition. The best tune we can achieve is a slightly rich one. This will ensure that the engine is protected and properly lubricated while giving us the best performance possible.

Most importantly, it is always better to err on the rich side since no damage will be done to the engine. When tuning, make sure that you start overly rich in order to ensure no damage will be done to the engine before the best tune is discovered.

Tuning by Color

Tuning by color is the more simple way to tune although in many cases, more risky. Seeing as our 2 strokes rev so high, it only takes a few cycles of bad tuning to seize or render your rings useless.

The color you are looking for in a spark plug tuning is a medium brown on the spark plug coil.

Tuning by (free-wheeling)

These carburetors require regular adjustment to ensure peak performance, and also to avoid an unsafe lean condition, which can prematurely damage your engine. If you find your top-end RPM’s have fallen off a bit, or if you experience lagging or surging, it is probably time to re-adjust the carb jets. To ensure max performance and engine life, follow this procedure:

Locate the low and high RPM jets on the side of the carb — they are usually marked with a L and a H. Turn both jets completely clockwise ( to the fully closed position). Rotate the High jet counter-clockwise to 1 and 1/4 turns open. Rotate the low jet counter-clockwise to approx 1 and 3/8 turns open. Now, if the engine doesn’t start, you may need to turn the low jet slightly one way or the other for the engine to start. Adjust the low jet as desired until the idle is where you like it. Turning clockwise (“closing” the jet) will produce a higher idle, and counterclockwise (“opening” the jet) will produce a lower idle (and eventually flood and the engine will kill, if you turn it too far). Open the throttle lever to full blast. Adjust the high jet until you get maximum RPMs (the highest pitch noise). Note: this will probably be really loud’. Listen for the highest-pitched whine to tell you where max RPMs are hit. Important: After you find the max RPM setting, turn the high jet counter-clockwise (“open” the jet) approx 1/16th of a turn to ensure you have sufficient gas/oil mix to cool the engine properly!

Tuning by ear (load-bearing)

While tuning by ear with freewheeling is relatively easy, it is not as accurate as tuning the scooter while you ride it. Tuning by ear is a learned skill since you will have to differentiate

between high, higher and even higher pitches. If you have access to a GPS or have a bike computer installed, that can be an invaluable tool to double check that you are hearing what you think you are hearing. During the process, you’ll also want to watch for tell-tale signs that you are running lean. See the troubleshooting section for more info about lean conditions.

You’ll need to find yourself a location to test where you won’t be bothering anyone by going back and forth a few times and you can let the engine wind up completely. Back roads and highway shoulders are great for this.

Turn the high needle so that you know it is running rich.

(open it at least a half turn farther than you think you need to) Make a speed run where you let the engine climb as high as it can in the RPM range.

(for the first run through, you should feel the engine hesitate in the upper RPMs suggesting a rich condition) Record your top speed, hum to yourself or otherwise document how fast you went. Turn the high speed needle 1/8th of a turn closed.

(this is about the width of a flat-head screwdriver) Repeat steps 2–4 until you feel the bog of a lean condition, stop going faster or start going slower. Re-open the high speed needle 1/8th of a turn and take another test run or two to make sure that you don’t feel the bog and that the engine is running well.

Troubleshooting

Stuttering at full throttle If your engine stutters at full throttle and cannot reach full RPMs, the high jet “H” is too far open. Close the jet by turning it clockwise slightly, and then ride-test. If no improvement, repeat until desired results are achieved. Once you have found a workable setting, open the jet approx 1/8 of a turn to ensure sufficient fuel/air mix.

Power drop during transition If your engine bogs out when transitioning from low speed, the high needle “H” is too far closed. Open the needle by turning it counter-clockwise, and then gradually close the jet until performance is as desired. , blocked exhausts, and very heavy engine loading can also cause bogging.

Sudden, fast idling If your engine idles very fast and stops running if you attempt to slow the idle, you may have an air leak in your intake. An easy way to check for leaks is to spray the intake manifold and area surrounding it with WD-40. If the idle changes after you spray the WD-40, there is an air leak. Common areas for leaks are between the manifold and the cylinder, and between the carburetor and the intake manifold. Inspect the manifold, carburetor gasket, and intake gasket for cracks or other signs of damage, and replace if necessary.

Idles but slowly dies If your engine idles but then slowly stops running, your low needle “L” is too far open. Close the low needle by turning it clockwise until a stable idle is achieved.

Hesitation during acceleration If you feel a hesitation during acceleration that feels like you are pumping the throttle, there might be something blocking gas flow. In order to check that you tank is venting properly, attempt to accelerate quickly and immediately slow down and open the gas tank cap. If you hear a suction noise, your tank cap is plugged and needs to be cleaned. Otherwise, your fuel filter or carburetor could be clogged.