I have an Exmark Lazer Z w/ 25H.P. Kohler Command vertical crankshaft engine. It has been a very reliable mower. I have app. 1400 hrs on engine. Several months ago, I noticed that very infrequently, the engine would start to bog down momentarily-this only happens about 5% of the time. Usually it happens when it's shut down hot and sometimes commences when I first power it up again after shutting down for a short time. It's like it isn't firing on both cylinders, sort of like it's not getting enough gas and a small amount of black smoke comes out of exhaust. After about 20 seconds or so it begins to gain R.P.M.'s and runs great, black smoke disapears. Like I said, this happens very infrequently but I wonder what is causing it. It does NOT burn any oil. I took it in to the dealer and he replaced plugs, etc. It still does it. I guess I don't blame him, it's one of those things like with a car or truck, if it does not run poorly 95% of the time and the dealer does not notice any thing wrong, he figures everything is o.k. Could this be a carburator adjustment/fuel mixture problem? Or head gasket problem? I just don't want to ignore it and something major happen. Any thoughts would be appreciated. P.S. I'll take it back to dealer when I get thge time, I'm hoping some of you guys can give me some idea to tell the dealer to check.