Husqvarna 266xp chainsaw

Most likely you need to adjust the "fast speed" adjuster screw on the carberator.
It's too lean and needs to be turned CC.
 
One screw is for the low speed , the adjacent for the high speed. The HS 
will be closest to the air filter, the LS closest to the block..
 
Adjust the HS to get it revving up, then the LS to get a good solid idle.
Fine tune adjustments putting the engine under load @ WOT.. 

IME, there is no "set opening" (for any machine). 
 
For EPA reasons, they set the machines as lean as possible. As engines wear in, 
they usually need more fuel(not a lot). Each requires an occasional tweeking.
 
You want a good steady idle that the machine starts hot with no throttle(adjusting the LS screw) .
 
For the high speed there are spec's for preferred RPMs.  I'll tune the HS adjuster 
being UNDER LOAD (spare log to test on)..  You may be able to get the engine to 
scream, but UNDER LOAD, it'll loose power (adjust, then test on the log).  
 
Turning in the HS adjuster, (as you now know), leans out the mix and it won't run or give a high idle.
Turn the HS adjuster out too far and it'll flood and load up.  You want a "happy medium", preferably on the richer side (definitly not leaner as you can burn up the engine).

SRTsFZ6
Most likely you need to adjust the "fast speed" adjuster on the carberator.
It's too lean and needs to be turned CC.
 
One screw is for the low speed , the adjacent for the high speed. The HS 
will be closest to the air filter, the LS closest to the block..
 
Adjust the HS to get it revving up, then the LS to get a good solid idle.
Fine tune adjustments putting the engine under load @ WOT.. 

Same thoughts.