If this engine is unmodified and sat for years and will only start on starter fluid The gas tank could have corrosion that caused a clogged up a fuel line and/or filter preventing the carburetor to fill with gas.
and/or the aftermarket air filter needs a bigger exhaust to go with it IF the proper size main jet is installed. I don't know what engine you use but a stock predator 212cc engine uses a #70 (.028"). the gx390's carb was barely bigger.
A bad carburetor gasket could also be the culprit causing a lean condition. Can't always tell if it's bad from being flattened out(used) and causing extra incoming air.
Or just do this: start the engine,rev it up high, and kill it while revved high. Now take out the spark plug and what color is it at the electrode? If it's on the darker side= it's on the rich side. Lighter side= its on the lean side.
I still think it's a fuel line/filter issue.
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