I have a Bobcat 743 that will run great for 15 - 45 minutes and then it acts like it runs out of fuel. Will restart with a little effort but will stall out again within a few minutes. It does blow white smoke constantly which is supposed to be caused by a stuck ring according to the guy I bought it off of who just put a new head on it but didn't touch the bottom end. When it is running it has power and seems to work fine. Any help in figuring this out would be greatly appreciated.