It's a 42cc. It will start when very cold and run for 20-30 seconds. I've checked the muffler. I've checked spark. I've cleaned the carb(everything looked very clean, including the screen). It's a Walbro. I tightened the adjustment screws all in, then backed out 1-1/2 turns. If I try to give gas, it dies. The piston and cylinder looked good from what I could see when I had the muffler off. I'm at a loss. The carb was rebuilt last year and ran like a charm. The last time I used it I ran it dry for storage. It's new gas this time. What could the problem be?