Why are my two-stroke grass trimmer and brushcutter not running to full throttle and why are they quieter than they used to be?

This is typically an indication of a restricted spark arrestor. This causes lower RPMs and quieter than normal running conditions.

Spark arrestor maintenance

What is the purpose of a spark arrestor on my grass trimmer and brushcutter?

Husqvarna recommends that the spark arrestor is cleaned after every 50 hours of use.