Have you cleaned carbs lately? Carbs are one of the most common issues. If it idles, try this trick. Pull wire off a spark plug, one at a time, then replace. Do each cylinder. Listen each time you do, that it makes about the same difference in how it runs. If you have one that you pull off that doesn't make much difference in the way it runs, that plug, cylinder, carb, wire, etc. would be suspect. It would not be fireing as well as it should. I would start with a new plug on that cylinder to see if it helps. We have also had bad caps that we have had to replace. They need to have a certain spec for resistance. Not exactly sure of spec offhand. If you haven't cleaned carbs in a while, most definately, get them cleaned. If you pull the carbs to clean, for maintainence, check the reeds for cracks or chips. As the others said, TORS can be an issue too. You can check that by pulling the wires at the carbs and plugging them into each other to complete the circuit. Make sure choke is going all the way off, by inspecting attachments at sides of carbs. My money would be on the carbs, and/or a little moisture in your fuel (get some Isopropyl). Good luck with your troubleshoot. Totallyamaha has a lot of info, including good instructions on carb cleanings. Let us know what you find.
