Hi, My Kohler CH25 suddenly started dying at idle and running rough with an occasional backfire. I suspected a head gasket, so I bought an inexpensive leakdown tester from HF. Both cylinders show the same thing, 50% leakage (moderate, according to the scale), hot or cold. Both plugs look fine, and an inline spark tester shows a good spark on both cylinders. There are no unusual mechanical noises. Stone cold, the engine starts and idles fine. After a couple of minutes at most, it starts missing and will die unless the throttle is advanced. At full throttle it can still do useful work, but doesn't put out full power. If I pull the plug wires in turn, the engine runs the same on either plug. The engine has less than 100 hours. The fuel is fresh. The oil and air cleaner were just changed. The one thing I noticed that seemed a bit odd was spurts of fuel from a horizontal brass tube in the carb throat. It sticks out from the front side of the bore. My service manual's exploded parts view does not show that tube, so I'm not sure exactly what it is. I assume it is the main jet, but it doesn't spray. It shoots a stream of fuel that seems a bit irregular. Thoughts?