I have a Yamaha 40hp (Mid-80's Yamaha 40ESG, 3 cyl, two-stroke) that has a hiccup at idle or when thottling it down to idle. I have messed with the mixture screws and I've tried adjusted the idle timing up and down. The only mods have been a shaved head and removing the oil injection. There are no leaks from gaskets or cracks. Plugs have looked good. I'm baffled. Any ideas? Thanks in advance.