My mower is a small 21" yard machine with a 5hp briggs motor. Junk I know, but this season it has been starting different. I prime it 3 or 4 times and then it starts right up and runs for about 10-20 seconds and kills. I have to do this about three times before it will stay running and then it runs fine till next time i mow. I changed the spark plug and took the bowl off and cleaned the parts with carb cleaner, the air filter seems fine also. If i know it is about to kill on start up, I can push the primer in a few times and it will stay running. Kind of wierd but i thought maybe someone on here has had the same experience before. Thanks in advance. 
