This is the best advice I have come across...


In general, engine manufacturers set their engines to run when they leave the factory. This does not mean, they are set at best needle settings, it just means, that the needles are adjusted in a way that the engine will start and run. Obviously, some needle tweeking is required if you want the engine to show what it is capable of.
Walbro carbs, as well as other carbs without floats, normally have two needles to set. The high speed needle labeled "H", and the low speed needle labeled "L". See the Walbro manuals http://mvvs.nl/manuals/wbmanual1.pdf and http://mvvs.nl/manuals/wbmanual2.pdf . It is important to remember that the low needle is always active, and that the high needle is only active avove 1/4 throttle, when there is sufficient low pressure in the venturi.
Here is the drill to start from scratch:
idle mixture
Open both needles two turns. This will ensure a rich setting.
Start the engine at slightly high idle rpm, and let the engine warm up a bit.
Adjust the idle needle (closest to the engine, and marked "L") for best rpm, and then open it 1/4 to 1/2 turn. This will be the setting to start tuning, and from here, the high needle can be set.
High speed mixture:
With the idle needle set about right, slowly apply full throttle. If the engine stays too rich, close the high needle a bit so the rich condition is cured, but no more than that.
This is the basic high speed needle setting, at which the engine will run without damage due to lean mixtures.
Keep this setting while running in the engine.
If the engine has run about five minutes, the plug can be checked. It should have a desert sand tan, or slightly darker. Not black, because that is an indication of an overly rich mixture, nor pale-white, because then the engine is too lean. This causes damage!
to be continued.
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Pe Reivers model engines
Dealer for MVVS and 3W engines, Mejzlik, MTW
www.mvvs.nl
www.prme.nl
www.mvvs-nl.com