I've got a stihl backpack blower that is not working properly. Hope anyone can help me. It is very hard to crank whether I choke it or not. I can pull and pull on the cord and eventually it will fire right up and run perfectly. I have started to switch out spark plugs when I am tired of pulling the cord and it seems to help sometime. when the choke is on i only pull the cord a couple of times to not flood it, and all i do is cut the switch on without giving it any throttle from the trigger. temperature doesn't seem to affect the starting. It seems to happen at just about every job. Any idea??