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Does it spit back through the carb?
Sorry I have to go out but will opt-out so someone else can help
Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, carefully remove the sparkplug and check the burn color of the ceramic at the tip end.
Still, two possible causes:
Let's try process of elimination; proposed further checks (if have not been done) on the possibility of over/under-voltage, regulator/rectifier and/or stator issue. With a volt/multi-meter, determine the battery voltage when:
The alternative is an engine issue.
Please post back results / observations of the above once tried.