Walbro carbs are an amazing little fuel delivery system that combine a fuel pump and air/fuel mixing system that feeds our model airplane engines the right amount of fuel and air under almost any RPM and aircraft attitude possible.
These RC airplane engine carbs require a periodic service or rebuild (at least once a year), especially due to the high alcohol content of pump gas sold today that damages gaskets and diaphragms. Note: When replacing carb soft parts from a factory service kit, choosing the light tan fiberglass-looking pump side gasket–over the black one—will prolong the service interval, but it may not draw fuel from the tank as quickly. We recommend cleaning the carb body using a heated ultrasonic cleaner and a 50% Simple Green solution to access the smallest low speed supply circuits.
The most common mistake guys make while tuning these carbs is setting them too rich, 4-stroke-sounding at various throttle positions. My advice to you is to experiment with the mixture screws: when the plane is safely secured on the ground, try turning the “H” or high speed needle screw at full throttle about a 1/8th turn at a time (CCW = rich / CW = lean) and listen to the exhaust note.
A “put put” tone indicates a rich mixture, while a steady roar indicates a correct mixture. Always wait 10 seconds after a change is made to evaluate your change, as the carb needs to make internal adjustments. Please remember that “L” low speed needle adjustments will affect high speed operation, so work on that aspect as well.
Once you have peaked the maximum engine RPM, we recommend that you take away 100 RPM by richening the high speed needle CCW for safe operation. An ideal setup is that you should be able to flick your throttle stick from idle to WOT and back down without hesitation or a flameout.
Without me yakking any further, here is a direct link to the Walbro website and the HDA Series Service Manual: