Stihl fs66

New to the forum and looking for some expertise.

I have an older Stihl fs66 brushcutter that will only prime with a shot of fuel/carb cleaner directly into the carb. New carb, fuel lines, gaskets. Impulse orifice is free and clear. When I do prime this way and get it running, it idles high. Exhaust is free and clear. This model has no vacuum bulb so I'm also wondering if I can retrofit with a carb that has one and run a return line to tank.