Walbro carburetor tuning
January 31, 2017
The balancing act between low and high needle seems to be the most difficult thing about gas engines with floatless carbs. It is important always to keep in mind that the low (L) speed needle is always active, and that the high (H) speed needle is only active above 1/4 throttle setting when there is sufficient low pressure in the venturi for the high speed needle to start drawing fuel.
A simple rule applies here:
If the engine starts with ease, the low setting is probably still is too rich. Lean out the low until starting without choke with a warm engine becomes impossible. That is the “too-lean” setting. From there, open up, but no more than 1/4 turn.
Another method according to Walbro instructions:
Always the low (L) speed needle is the one closer to the engine and the high (H) speed needle away from it. At slightly high idle (about 1800 rpm), find the L-needle setting with best rpm, by finding the lean rpm drop and the rich rpm drop. Best setting is in between.
Then set the high needle for best rpm. Find the rich rpm drop of the high needle. Stay just clear of that rpm drop. Now a cold engine should not accept throttle until well warmed up. Once warm, throttle response should be crisp. If not, richen the L-needle slightly. This will improve the hot starting, and also add to the idle shake. This is normal, and just the thing that two stroke gas engines do.
Fine tuning the L-needle should be done gently. Turning the screw a hair makes a difference. Using 1/4 turns, or even 1/8th turns will probably overshoot the ideal setting. In some carbs with steep idle needle taper, 1/16th of a turn is too much.
To summarize: Because at full throttle, both needles contribute to the mixture, you can run the engine with lean idle and richer main needle for good mid-range, but starting will suffer. You can also run the engine with rich idle, and leaned out main needle, but mid-range will suffer and burble. A larger prop will run cleaner at mid-range. Too large a prop will make a good tuning very hard to do, and it will be almost impossible to get full throttle just right.