Tuning a Walbro Carburetor

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 needles.

The carburetor is provided with 3 adjust screws they may need a little adjustment depending on the temperature, atmospheric pressure (altitude), etc. of the area where the engine is used.

carb

Idle Screw:
Turning this screw clockwise increases the idling R.P.M. Turning it counterclockwise decreases the idling R.P.M.

Low Speed Needle:
This is the fuel adjust screw (not the air screw). Turning this needle clockwise makes the mixture gas leaner and turning it counterclockwise makes it richer.
Set this needle at a position which is 1/4 open from best mixture (maximum R.P.M.) position.

High Speed Needle:
Turning this needle clockwise makes the mixture gas leaner and turning it counterclockwise richer. Set this needle at a position which is 1/8~1/4 open (counter clockwise) from the maximum R.P.M. position

 

WT 1048 Tuning Procedure:

  1. Locate the low and high RPM needles on the side of the carb – they are usually marked with a L and a H. Turn both needles completely CLOCKWISE ( to the fully closed position).
  2. Rotate the Low needle counter-clockwise to 1 and 1/4 turns open.
  3. Rotate the High needle counter-clockwise to approx 1 and 3/8 turns open.
  4. Connect your water system to a water supply and turn on the water flow.
  5. Now, start the engine. You may need to turn the Low needle slightly one way or the other for the engine to start.
  6. Adjust the Low needle as desired until the idle is where you like it. Turning clockwise will produce a higher idle, and counterclockwise will produce a lower idle (and eventually flood and the engine will kill, if you turn it too far).
  7. Open the throttle lever to full blast. Adjust the High needle until you get maximum RPMs. Note: this will probably be quite loud..Listen for the highest-pitched whine to tell you where max RPMs are hit. After your find the max RPM setting, turn the high needle counter-clockwise  approx 1/16th of a turn to ensure you have sufficient petrol to cool the engine. Don’t do this last step to a brand new engine, run the motor in for at least a few tanks of fuel before the full blast tune.