I've got a 25 hp Kohler horizontal with arund 1300 hours on the clock. I haven't been running it much lately with the dry weather here and it suddenly developed a potentially severe problem. First start of the day, the last 3 out of 4 starts it has started right up but only been running on one cylinder. It runs for 15-20 seconds and then quits. At that point the engine will spin over very easily, as if there is no compression. It usually backfires once when it stalls, then poops and pops until it finally lights up again. As it is cranking you can hear compression starting to build up and the starter takes a load. Finally it lights up on one cylinder again, barely able to keep turning over, and gradually smooths out and the other cylinder kicks in. There is no odd noise during all this other than what *may* be lifter clatter while cranking. One it runs on two it shows no noticeable power loss or lack of RPM. Keep in mind that this came on instantly, one start was normal, the next was choke & puke. I haven't checked compression yet but I have held my hand in front od the exhaust and there is virtually no air coming out of the pipe when it is cranking with no resistance. Any ideas of where to start, other than a compression test to confirm what I already suspect?