I have an 2-cycle stihl weed trimmer and blower that have recently seemed to have problems. The machines will start... but when I pull their triggers to get them going they seem to "bog" down and will not spin or blow very strongly. Sometimes they will even just shut off. They both require "mix" for fuel... and the fuel I use is good, clean, and I use the 50:1 ratio stihl engine mix at the right ratio of oil with gas. Any ideas on the problem? Thanks.