Too lean= not enough gasoline in the gas/air mixture. Caused by any of several things, least of which is the dreaded "air leak". Can cause hot running and scuffed pistons among other maladies. A lean condition will manifest itself as smooth running, faster than normal running, but with poor acceleration. Another simptom, and common when an air leak in the intake is encountered is a run away idle or an engine that take several seconds to wind down when revved (clutch disengaged).
Too rich= too much gasoline in the gas/air mix. It can cause rough running, fouled plugs and carbon/coke build up. Not as dangerous as too lean. A lot of the china in-frame engines come with too large a jet- causing a rich condition even with the clip in the top notch. Very rich conditions cause the engine to four cycle, where it hits every other revolution and sounds like a four stroke (sorta).
There is a thread dealing with delo'rto jet for replace ments, a quick search will find that.
There is a screw on the carb commonly refered to as the "mixture screw" in a lot of the instructions....it just an idle speed adjustment, as the re is no mixture screw. All mixture adjustments are done with the "C" clip on the slide needle. Down for richer, up for leaner. Start at the second notch from the top, all models.
When setting up an engine, if you have a doubt about where it's running, always check your plug. Dark black/sooty/oily is too rich, though oilyness is cause by other things as well. Too lean and your plug will be white or light grey. Just right is anywhere from chocolate brown to carbon around the edge, grey center elctrode...but that's on the lean side, chocolate or moderately black is where you want to be.
Always error to the rich side of things when in doubt.
