Charon
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Coil readings sound about normal. Usually, the coil fails "open" and the machine won't run at all. Your description of holding the throttle wide open and getting rough running and flames sounds to me like you are running up against the electronic rev limiter said to be in the ignition box. When you hit that, it will cut the ignition until the revs drop again. During the time the ignition is cut, the engine will continue to inhale a combustible mix of fuel and air, and expel that mixture into the exhaust pipe. Then, when the engine speed drops enough, the rev limiter will allow the engine to fire again. The mixture in the exhaust pipe will ignite as soon as the first "fire" exits the exhaust valve. The result will usually be a huge blast. It's an old stunt used since, probably, the Model T - turn off the ignition for a second or two, then turn it back on to make a loud bang.
It could still be the coil. If the coil is failing, but not completely dead, it could be opening up as it gets warm. It could be getting intermittent until it finally gets hot enough to quit completely. You could try making your resistance measurements while heating the coil with a hair dryer. It could also have gotten a bad spot in its internal insulation allowing it to arc internally under high throttle conditions (when the cylinder pressures are high and thus the required spark voltage is also high). That failure will not show up using an ohmmeter. The only real way to find out is the old substitution method of putting another coil on the motorcycle and seeing what happens.
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