Customer complained of stumbli
Customer complained of stumbling and dying on acceleration. He had this same problem in the past and took it to a shop that rebuilt the carbs. He said that it ran fine for some time but the problem was back. I pulled the plugs and right away noticed they were scorched white. Compression was good and even on all four. Ignition tested good at 7/16 gap of the tester. Spark plugs indicated fuel starvation to me so I checked the tank pick-up and anti-siphon valve OK. I replaced the hose and bulb assy and fuel filter. I then disassembled the fuel pump expecting to find stiff diaphragms but they were ok as were the check valves. I went ahead and replaced the pump anyway because the fasteners attaching the check valves were showing signs of corrosion. Fuel pump pressure and flow was then checked and ok. Assuming the carbs had been redone I left them alone. The test run revealed the same problem. Next I removed and completely disassembled the carbs. They were clean and properly set up (float adjustment and all jet sizes per Yamaha) but I went ahead and purged all passages with carb cleaner followed with compressed air. The next test run revealed the same problem spark plugs still scorched white. I next pulled all four main jets and inspected them with an otoscope and found them free of any obstruction (no gum varnish nothing). I drilled all mains .003 oversize and that took care of the problem. Can any one offer an explanation as to what caused this problem?
Customer complained of stumbling and dying on acceleration. He had this same problem in the past and took it to a shop that rebuilt the carbs. He said that it ran fine for some time but the problem was back. I pulled the plugs and right away noticed they were scorched white. Compression was good and even on all four. Ignition tested good at 7/16 gap of the tester. Spark plugs indicated fuel starvation to me so I checked the tank pick-up and anti-siphon valve OK. I replaced the hose and bulb assy and fuel filter. I then disassembled the fuel pump expecting to find stiff diaphragms but they were ok as were the check valves. I went ahead and replaced the pump anyway because the fasteners attaching the check valves were showing signs of corrosion. Fuel pump pressure and flow was then checked and ok. Assuming the carbs had been redone I left them alone. The test run revealed the same problem. Next I removed and completely disassembled the carbs. They were clean and properly set up (float adjustment and all jet sizes per Yamaha) but I went ahead and purged all passages with carb cleaner followed with compressed air. The next test run revealed the same problem spark plugs still scorched white. I next pulled all four main jets and inspected them with an otoscope and found them free of any obstruction (no gum varnish nothing). I drilled all mains .003 oversize and that took care of the problem. Can any one offer an explanation as to what caused this problem?