How to adjust my throttle cable on my backpack blower br 550 sthil

I will try and help with your question. Could you please tell me if the throttle on your carb is fully activated when your trigger is squeezed. Or is there still more travel on the carb? I'm looking at each possible fault individually, In this case, many back pack blower have adjustable throttles to take any slack out of the throttle cable, and yours may have slipped. But I am unfamiliar with your machine so require a little more information.

BR 600 inlet valve = 0,10 ± 0,02 mm also 0,10 ± 0,02 mm exhaust valve

Sorry only in Dutch

well there are several Torx bit screws holding just about any mordern Stihl product together. most likely need to remove the recoil first and then the four perimeter screws... just above the gas tank deep in there you will see the bottom 2... yes a pain !!!

4l60 ???

rebuild it

The lever may have broken because the cable is stuck due to corrosion or a bent/damaged cable covering. Can you take off the air cleaner and work the throttle manually at the engine? You might have debris on the carb holding the throttle closed.
Also try removing the lever and work the cable end with your hand or a pair of pliers. The amount of force should be quite light. If the cable effort is too high the cable is likely in need of replacement.

get one from an old bike ...
i would if you need just the end fixed use a choc box connector
used for connecting mains wires etc together
has two screws
strip off the plastic and you have a small bar with two screws
that can retain the cable inner

the fuel system is blocked up
perhaps the plug is bad
and the air filter will need replaced

or the fuel tank vent is also blocked up

replace fuel filter
air filter
spark plug
and clean the carborator jets etc...
change/change out the oil and measure the amount you put in
by the manual info look on the sthil site for the manual

http://www.mediafire.com/download/i7tdlr02j8awfm9

You need to buy a new cable, clarified by Ryobi Tech Support.