Tune Your Carb

This article will show you how to adjust the jets on a carburetor. The images in this article are of a typical Walbro carburetor, but the same concepts apply to nearly any carburetor. (WA-167 / WT-603 / WT-668 / WT-257 / WT-813, etc)

Caution: Your engine lubricates itself with the oil in the gas that enters the engine. If you make your mixture too lean (too much air, not enough gas) you can damage your engine. Even though it may run great, it could be getting too hot. It is a good idea to err on the side of richness (too much gas, not enough air) just a little bit.

Adjust often

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.

Common problems fixed by tuning

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 jet "H" is too far closed. Open the jet by turning it counterclockwise until you are at least 1 1/2 turns open, and then gradually close the jet until performance is as desired. Poor engine compression, blocked exhausts, and very heavy engine loading can all 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 carb and the intake manifold. Inspect the manifold, carb 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 jet "L" is too far open. Close the low jet by turning it clockwise until a stable idle is achieved.

Tuning steps

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images borrowed from davesmotors.com

To ensure max performance and engine life, follow this procedure:

Tune with the back tire off the ground

  1. Locate the low and high RPM jets "L" and "H" 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).
  2. Rotate the Low jet "L" counter-clockwise to 1 and 1/4 turns open.1
  3. Rotate the High jet "H" counter-clockwise to approx 1 and 3/8 turns open.2
  4. Now, start the engine. You may need to turn the Low jet slightly one way or the other for the engine to start.
  5. Adjust the Low jet "L" as desired until the idle is where you like it. Turning clockwise (closing the jet) will produce a higher idle and counter-clockwise (opening the jet) will produce a lower idle. If you open the jet enough, the engine will eventually flood and stop.
  6. Open the throttle lever to full blast. Adjust the High jet "H" until you get maximum RPMs. Caution: This will probably be very loud. Listen for the highest-pitched whine to tell you where max RPMs are hit. Warning: After you find the the maximum rpm setting, turn the high just counter-clockwise (open the jet) 1/8th of a turn. Always do this. This will ensure that you have sufficient gas and oil in your mixture to keep the engine lubricated and cool.

Fine tune while riding

You may need to adjust both the "H" and "L" screws once you begin riding, depending on engine loading, altitude, humidity, etc. The trick is to find settings that work well for you, and stick with them.

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