Welcome to the Forum,
You came to the right place for info regarding your GL500.
Please add you bike model to your signature line that way other members can see what type of bike you have for any further questions.
Did you clean the carbs after you got the bike? After sitting for 2 years something that was already in the carb may have jarred loose
and clogged a jet or orifice so the in-line filter would not have helped for that. You should take the carbs off and give them a thorough cleaning.
Pull your spark plugs and see what condition they are in.
I'm sure other members will chime in with their thoughts on the matter as well.