My 15HP Kawasaki FH-451 is surging slightly, the rpm goes up and down, and it does it even when fully warmed up. Sometimes it does not do it. What do you think the problem might be? Spark plugs? I took one out and it was very dark in color, and dirty; possibly not gapped right too. Could new spark plugs fix it? It is likely a high-hour engine too (there is no hour meter on it). I hope it will be something simple to fix. Do you have any advice?