Are you using the choke to start it from cold? Getting just the right amount of choke can be difficult, so most people follow the chainsaw system. You close the choke completely, pull it a couple of times until it burps (runs for a second then stops), then you half-open the choke and it should start first pull.
If there is no way to get it to start using that technique, there is a good chance that it needs new carburetor diaphragms. In general, as the diaphragms get older the starting technique gets more critical.
There will be three screws with springs under their heads on the carburetor. One will be marked "H" (high speed mixture adjustment), one will be marked "L" (low speed/idle mixture adjustment) and one won't be marked but is the idle speed adjustment. You will be able to see that one move the throttle butterfly when you adjust it - it acts as the throttle stop.
