Has firing but wont start

Sorry I meant to say dies when you let of f of the button

Check if its getting fuel and air, you need All 3 to make it run, if you have all of that (and going on the its firing concept) all that need to happen is fuel goes to carb, intake turns it to vapor, spark plug fires and ignites the fuel vapor and that causes the piston to go, so if its firing and all the above are good you should check the points and condensor, it should turn over for a few seconds and backfire if its not backfiring its likely not getting fuel, try to spray some ether(quik start or just some gas directly into the carb to see if anything hapens, You may need to take it to a shop or have friend with some mechanical background help ya. good luck Rotticus,
p.s. in the future be more specific and we can try to get more details for you, also google the bikes make and model and you should find some schimatics.