I bought a reconditioned mc cullough 42cc chainsaw w/18 bar that ran real good when I got it, only to fail when I wanted to cut something. It will start and idle, and then after about 10 seconds of wot it acts like it's starving for fuel. After priming it,and pulling the recoil it will start and idle as long as you don't touch the trigger, but when you squeeze the trigger to wot it will run strong for another 10 seconds and fall on its face again. Any ideas?