As said, check the spark quality, check for vacuum leaks, check fuel flow through the drain plug, try opening the main fuel adjustment screw 1/4 turn at a time.
If it still is too lean, the carb will need to come off and be opened up and cleaned. If it has been sitting with ethanol fuel in the bowl, chances are it is clogged internally. Inside there will be various fuel jets. The main jet is the most common to be clogged. Dried ethanol residue is difficult to remove, carb cleaner will not get it, it has to be scraped out with a small wire.
Don't be intimidated, the carb is simple and field serviceable. Lots of online information, parts break down, how to manuals, and Youtube videos to walk you through it. Just work over a clean surface so not to loose anything.
If the inside of the carb is dirty, chances are the tank is also needing cleaning. Best to take it off, dump it, rinse it with E85 gasoline if you can find some. Be careful handling and bumping it, as it is probably got some thin rusty places. If it is severely rusted and flaking, a replacement may be a consideration.
Inline filters are not a good solution. They don't work well with gravity systems. A clean tank, the screen in the sediment bowl, and the screen in the carb elbow fitting will be sufficient.