Start with the basics.
Even though this may end up being a fuel issue.. since it is so easy.. go ahead and check spark at all 4 plugs. if spark is good on a spark checker tool, or holding wire near block.. then go ahead and pull plugs and check them. never know.. might have a fouled plug. clean and regap the plugs. if spark was good and didn't need cleaning, or clening provided no change, then move on to fuel.
make sure you have a good flow thru carb. if so, screw the main in and count.. if it is less than 1.5 i'd say lets go back out to 1.5 and then try 1/8 turn out testing throttle and richen it a bit and retest.. if you get past 2 turns out and still bad, yet runs ok with choke, ad otherwise has good power, I'd reccomend to pull the carb down for a good cleaning.. dip then spray out and run holes with fine wire / guitar strings.. etc. go in with a new gasket. make sure choke flapper is good and not loose.. sme with butterfly.. etc.
make sure you clean the gasket surface at carb halves and carb to manifold.