k i'm posting this cause i just had this issue and i searched high and low didnt see much on it... my bike would not idle and pop and die... i tried everything from adjusting valves to cdi, coil, carb kit... after all that work i tore my carb apart so mant times i just noticed the idle mixture screw. it looked like most i've seen with the end stoppin with a flat surface but still tappered.. i looked in the carb with a tiny light and noticed a tiny piece of brass at the bottom.so i did a check but blowing air in the hole but you have to cover the piot jet in the carb as well and nothing came out the tiny hole in the bore(after the slide just before the intake). i trie to drill it out with a very tiny drill and got air out of the hole but after that it would idle but had a very small limit of adjustment due to drilling the hole aka eliminating the taper to get the broken end of the needle. the needle should come to a point not a dull end but a point almost sharp enough to pierce ya... i was lucky and my local cycle salvage had a excat carb and i swaped my carb kit over and the needle was perfect and it runs like a dream and would idle all day if i'd let it... i hope this hlps some people out

 

That is a common problem for people who really crank the adjustment screw into the bore instead of letting it lightly seat. 9/10 times it with either break off like that, or mess up the taper.

 

i got my bike from a barn where it had been sitting for six or more years...so when i screwed it out it must have broke...i've rebuilt numerous carbs but this is the first time i've had that issue.. yes people turn them in way to hard. you only need to tun it in lightly till you feel it start to seat.