I have borrowed my father inlaws case skidster 1816b. It has been running great until the last couple of weeks. The engine has been running rough at times. When I start it it runs great for about 10 - 20 minutes. Then after it warms up, for some reason it does not want to stay running and stalls out. If I pull the choke all the way out ( on ) it runs fine for a few minutes and starts to run rough again. Then I push the choke back in ( off ) and it will run fine for a few minutes then it starts to run rough again so I pull the choke out ( on ) again and it run fine for a few minutes. I am getting tired of doing this I would like to fix the problem. I have put a fuel cleaner and water remover in the gas tank thinking this may help but it made no change good or bad. Then I changed the fuel and air filter thinking that might be the problem, but again no change still runs the same having to push and pull the choke. Does anyone have any ideas as to what might be wrong, and how to fix it. I asked my father inlaw if he had any ideas and he said it might be the fuel pump, he was not sure. Can anyone help. Thank you.