how can i clean my lawn mowers carberator?

how can i clean the carberator on my 21" sears craftsmen 6.75 horsepower rear discharg mulcher
model #917.388610.
it will start, run about 10 seconds and then cut off again.
after about 30 minuetes of doing this it will finally crank up and run for me to cut the yard. sometimes it will cut off again while mowing the yard and not crank right up futil it has been cord pulled crank cut off cord pulled, crank cut off process for about 30 more minutes. help i need to save the money by doing t