1 pointthe 3 most common things i have seen cause erratic running on these is 1) a plugged fuel tank vent (either improper routing or our friendly mud dauber hornets). 2) is a tiny hole in the fuel pump diaphram (at idle there isn't enough fuel draw, and at WOT there is so much of a pulse it can overcome the leak), and 3) is a delaminating intake manifold. they can delaminate internally as well as externally, so its not always easy to see. the test for the manifold leak is to have the machine idling (engine just warm enough to run without choke), and spray the manifold with water while grabbing the carb moving it in every direction. / its really difficult when you have 2 systems that can be at fault (in this case both electrical and fuel). the fuel system is easier to diagnose than the electrical, so go there first. i will add that most (but definitely not all) of the electrical problems i saw in the 40 years of doing this was related to a bad ground. a quick way to eliminate that is to run a single wire from a GOOD CLEAN (meaning sanding, filing,etc. all of the paint off) chassis ground, to a CLEAN bolt on the engine case, to the battery - terminal, and then make certain the voltage regulator and CDI box and ignition coil all have CLEAN & TIGHT contacts as well. oh, something else just popped into my head, i have seen a few cases where the ignition coil connector was worked oversize from vibration causing intermittent ignition at specific rpms. tightening the contacts up by either replacing or bending them to increase the tension fixes that. its a lot of things, but i hope it helps.
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