My Cub has always been a little unpredictable, starting on the first or second pull sometimes and sometimes taking 15-20. My procedure has been to turn the gas on, check the oil, do my pre-flight, prime with three pumps with the last one giving resistance, pull it through 4 times then make it hot and swing the prop. Recently I started priming before the pre flight thinking the gas might vaporize and be better pulled into the cylinders with a little more time. For three times in a row it started on the first pull doing this and I thought the problem was solved until yesterday when it failed to start after a lot of prop swinging. It has new plugs and the carburetor was rebuilt about 50 hrs. ago. It runs and idles fine once started. The amount of gas in the tank may be a factor it seems to do better when the tank is full, yesterday it only had about 5 gallons in it. Any suggestions would be appreciated.