I have a XL120XLS GM 6 cyl that is in good shape, but I've been having problems with it the last 6 months. I can get it to start but sometime it idles rough and sometime it doesn't and idles OK, but the kicker is that if I give it gas it stumbles and wants to die, Sometimes I can get it to clear up and rev it a bit and sometimes that works. However I can let it kind of idle awhile until it gets warmer and somtimes that helps. I have change tanks, rebuilt the fuel lock and the vaporizer and it pretty much runs the same. , If it dies and I leave the key on it will backfire when I try to start it again. It is driving me crazy, any help??