Have you checked compression? (With motor warm and WOT.) Also pump primer bulb to make sure carb float bowls are full and that you don't have a problem with vapor lock (more likely with E-10 gas!) Problem is most likely in ignition system. Coils expand when they get hot, so if there are breaks in the windings, the coil will work fine when cold, but output voltage will drop when coil gets hot and expands, opening up cracks in the windings!
Suggest you make up a spark checker . . . get a PVC cap, maybe 3-4" ID, and get seven 1/4-20 SS screws about 3"-3.5" long, a ~12" length of wire, an alligator clip and a 1/4" ring terminal. Attach alligator clip to one end of wire and a 1/4" ring terminal on other end; drill 1 hole in center of cap, tap with 1/4-20 thread and install 1 screw into it with ring terminal/wire under head of center screw on outside of cap. This will be a ground wire. Drill 6 more radial holes around outside of cap, spaced every 60 degrees, and tap with 1/4-20 threads. Spray inside of cap with flat black paint. Cut the heads off 6 screws and install in sides of cap; use a 7/16 nut, which fits a 1/4-20 screw, to set a 7/16 gap between the center screw and each of the 6 radial screws, which should extend ~1 1/2" outside of cap.
To check entire ignition system, clip ground wire to engine block, pull all wires off plugs and attach each one to a stud on outside of cap. Hook up flushing attachment and turn on water. How have someone crank motor and watch to see if a bright blue spark jumps from each of the 6 studs to center ground screw. With healthy coils and power pack, the spark will jump a 1/2" gap, so if the spark on any of the 6 studs won't jump a 7/16" gap, you've got a problem on that coil and/or power pack! I've found bad coils and power packs with this tool. If you don't get ANY sparks, it could be a problem with the sensor coils in the stator assembly which trigger the ignition system, or a problem with key switch.
Hope this helps! Keep us posted on what you find!
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