This is what DDM says to do to set your carb
How should I adjust the jets on a Walbro Carburetor (WA-167 / WT-603 / WT-
668 / WT-257 / WT-813, etc)?
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:
1) 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).
2) Rotate the Low jet counter-clockwise to 1 and 1/4 turns open.
3) Rotate the High jet counter-clockwise to approx 1 and 3/8 turns open.
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 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).
6) Open the throttle lever to full blast. Adjust the High jet until you get maximum RPMs. Note: this will probably be REALLY LOUD. Listen for the highest-pitched whine to tell you where max RPMs are hit. 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.
Note: The above are just general guidelines. You may need to adjust both the H and L screws once you begin driving, depending on engine loading, altitude, humidity, etc. The trick is to find settings that work well for you, and stick with them.