Grounding the spark plug leads or at least through some kind of spark checker is highly recommended. As they always say, electricity is lazy. If there is too big a gap for the spark to jump, it will eventually find an easier path through the insulation in the coil winding, someplace. Japanese ignition parts are absolutely as good as it gets, but nothing is THAT good, for long.
Does the owner report that it stalls at low speeds? If not, I am thinking your garden hose test may not show anything but three running cylinders…..
Very wise move to observe the throttle action before doing anything as rash as a carb job on that thing.
Don’t forget that the “fuel system” includes the tank and line! One time, I saw a Yamaha that idled well and would run mid-range just fine; but it wouldn’t go fast. The owner had attached the fuel line to the tank backwards. At low speed, there was enough fuel getting by the backwards primer ball valves to keep running! I have also seen many plastic tanks, that have a plastic withdrawal tube. The ethanol damages the tube, which causes air to leak into the system at the top of the tube. That could definitely cause high speed problems. remember that trouble shooting is a matter of “divide and conquer.” It would be best to take a ride with an entirely different, known-good tank and line and see if he gets a different result in speed.
Long live American manufacturing!
