I'm working on a murray with a Techumseh HSSK50 which had been stored outside in the weather for years. I replaced the rusted out carb and it will run if i use the electric starter and hold it for about 20 seconds then it will run and sound good for about 5-10 seconds before it fades out and dies. I replaced the coil and spark plug as well and it still does the same thing. One time i got it to run for 5 minutes and i took it up and down the driveway. I shut it off and it would not start again without the electric starter and it would die again after a few seconds. I'm wondering if it could be the valves? I've never opened one of these up and I'm out of ideas. Any assistance is appreciated.