An FS 45 is a 2 stroke 45cc power head made by Stihl, only valves are the check valves in the carb & primer.
At 90 psi you are border line for compression on most Stihls.
pop off the muffler & check the bore & rings.
try some squirt down the plug hole with a hot plug ( 10 minutes of a hot air gun will do ).
To check the carb, remover it from the engine but leave the fuel lines attached.
Turn it sideways and blow through the carb with high volume low pressure air.
You should see the fuel being drawn through the fuel lines and also see it blowing out the carb.