sounds like one of them stupid things that ya never think about . try a different spark plug , ya ya i know what does the plug have to do with anything . just try it have had some wierd stuff happen over a bad plug ,check spark is it good and strong .it still does it? well get it started and grab the carb spray and shoot some around the gaskets from the intake on the head to the carb . ya dont need to soak it just get some around the gaskets . does it improve ? yes then there is your bad seal . no? well then back to the carb do the throttle/choke shafts fit good and tight ? there should be no side to side movement in them if so there is a problem , it will suck air in and mess up running . if there tight then well only a few things left to try , take and turn the motor over by hand . removing the recoil makes this easier . get to the compression stroke and listen do you hear any air coming out of the ports ? compression seem good and no escaping air . then well only thing really left the carb is 1 junk or 2 you did not get all the crap out of a port or passage some place in it .
i know its kinda a strange list , but i have had carb problems that were in the coil and coil problems in the carb . and some times is just crappy compression . my tec hs40 acted up bad and got to where it would not run for nothing, carb ? nope spark? nope timing ? bad fuel ? afraid not the exhaust valve went out of spec had me pulling that dam rope for hours and hours .:doah: