Walbro carburettors are almost standard on most all two-cycle engines. Two-cycle engines require that oil is mixed with the gasoline fuel. Failure to mix the correct oil in the gasoline will result in permanent engine failure and damage. Adjustment of the carburettor may have to be performed on a periodic basis and most definitely after the simple device is rebuilt. Adjustment will not take long and the small screws may have to be "tweaked" after the engine has warmed up and been running for a few minutes.
Identify the two-fuel adjustment screws on the side of the metal carburettor. One screw is labelled "lo" and the other is marked "hi". The "lo" fuel screw is for when the engine is idling. The "hi" fuel adjustment screw regulates the fuel when the engine is at full throttle.
Turn both of the screws, gently, in a clockwise direction to seat the base of the screw inside the carburettor. Do not tighten the screws with any form of a forceful nature. When dealing with these small carburettors "finesse" is a key word for manipulation.
Open the fuel adjustment screws in a counterclockwise direction one and three-quarter turns to two full turns. This is the starting point for all Walbro fuel screw adjustments.
Start the two-cycle engine. Allow the engine to warm up for approximately one minute to two minutes. The engine may be running a little rough, that is normal until final adjustments are made.
Hold the throttle wide open for full engine acceleration. Adjust the "hi" screw until the engine smooths out. In other words, the engine should not be missing or have any hesitation. In most cases the "hi" fuel screw will not have to be turned more than a one-quarter turn in either direction.
Allow the engine to return to an idle operation by releasing the throttle. At idle the engine should be running smoothly. Quickly depress the throttle again. If the engine hesitates open the "lo" fuel screw to a quarter turn counterclockwise. Perform the release and depression of the throttle a few times. The engine should go from idle to full RPM in a smooth progression. If the engine pops heavily at idle, when coming down from a full throttle, the "lo" fuel screw is delivering too much fuel. Turn the "lo" screw in a clockwise direction using no more than quarter turn increments.
- Follow the engine manufacturer's specifications for mixing the correct oil with the proper grade of gasoline.
- Many times, poorer grades of mix oil and gasoline will disrupt any adjustments made to the carburettor.