I have a 20 hp gas Walker. This problem has just started. After I run the Walker for about 15 minutes or so, The engine starts acting like it is out of gas. I have to play with the choke, moving it back and forth, just to get it to move a little bit. Then, the engine will die. If I let it sit for a little while, the problem stops till I run for another 15 minutes. Please Help!