I have an Exmark Viking, 15 years old, with thousands of hours (maybe 6 or 7). The engine is a Kohler Pro Command, 15hp, recoil start, single cylinder. The machine was repowered with the same engine a few years ago. My expectation is the first engine had 2,500 to 3,000 hours, and this one has at least 3,000 hours. For many years, it was run very hard all season long. I expected this second engine to fail before this, but I think it now has a problem. It uses no oil, or perhaps just a little between changes (about 40 hours, new filter every 80, Shell Rotella 10W-30). But, I think the head gasket has just begun to leak. When cranking, I hear a leaking sound, a pffffft sound (sorry - the best I can do). Also, the engine starts much harder than a couple of weeks ago - then it was starting first pull, nearly every time. Now, it takes much choking, and many pulls. After a few pulls, with full choke, I see a bit of smoke come from under the front cowling, in the area where the exhaust pipe come, before running over to the pillow muffler. It still has power, but nothing I've done recently has required much power. I'm nearing the end, with final cuts, without much grass. All the heavy leaf mulching work is finished. Also, at WOT, it seems to be running at higher rpms that before. I'm throttling back a bit from full stop, thinking it might be over-reving. On engine shut-down, I also hear the leaking sound for the last few revolutions. These are pfffft sounds like on initial cranking on starting. I have never had a head gasket fail before. From my description, do you think this is my problem? If so, is a replacement a difficult thing to do? With a side-valve engine, taking off the head is a simple task. But, I'm unsure where the valve come through, how to deal with the OHV cover, and what is behind that cover. I have no service manual. Also, I don't have much in the way of diagnostic or test gear. As said before, my season is nearly finished. I still need it for a few mowings. So, I'm hoping it will continue to hang together for a short time longer. If a gasket replacement sounds reasonable, this is an off-season task. I know that I could repower with a new engine, but that seems a stretch for a machine this old. Otherwise, the mower remains in good condition. I know the hydro pumps are vulnerable to failure (I've replaced one of them a couple of years ago). But, I don't want to spend $750 on a new engine on this old piece of equipment. Any thoughts from the mechanics in the crowd? Some of you have been very helpful in times past, for which I appreciate. And, some of you have followed other repair efforts of mine, so know my capabilities -- this repair may be beyond my capabilities, or maybe not. Thanks. R.