Tiller will start but won't run.anyone own any thinking?

I have a 5hp Craftsman tiller that I have never have any problems with until recently. I can start it, but it lone runs about 5 seconds and later quits. I have checked & replaced the gas lines since I believed that it was not getting any gas, but that didn't sustain at all. I know it is getting gas and the spark plug is new. Do you own any ideas of what is going on with this tiller?
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Answers:    I agree that it sounds similar to it's not getting enough gas. If it has a gas filter, check/change it. If it have a vented gas cap, be sure the vent isn't clogged. To check this, try to start and run it beside the cap off. Do this fussily so as not to splash or spill gas. If there is a 'bowl' [sometimes glass] under the carb, remove it [shut bad gas valve first], clean it out, and reinstall it.
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Just a possible notion. Are you using fresh gas. We have a gas can that sits in our garage & other use it in our lawnmower. If it gets hot plenty, you can end up with wet in your gas & your tiller won't run. Maybe you could try using new gas-if that's not the problem, it could terribly well be the carborator. Hope this helps.