Your machine has a rotary-valve diaphragm type carburetor. These carburetors typically only have an adjustment for the IDLE speed with an available option to control the main jet with an adjustable mixture needle.
http://www.echo-usa.com/Products/Blowers/PB-250
Here is the info on the Zama rotary-valve diaphragm carburetor.
http://www.zamacarb.com/prodlist.html#RB
Some have a small adjustment needle in the center pivot of the rotory part, under a brass plug...but most have nothing. Many end users and techs are complaining about the non-adjustable carbs. They are made this way to meet the more stringent EPA emissions codes that are getting tougher each year.
They also state (as I haven't worked on one yet), that you can remove the plastic plug on the flat side of the carb flush with the purge plate, and access a small adjustment screw and many of th units have a mixture jet located there.
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There are adjustment screws in most if not all carburators. Its cheaper to take to a small engine shop to have adjustments made then paying fines that could accure from doing your own repairs on epa regulated equipment.
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