On these it's ignition key - tends to play up on the starting side and once off it's off... Had a couple of the freaking key switches filled up with dirt/water/crap and needing to be pulled apart cleaned up and put back together. I've got to get one of the little barb to barb inline filters for mine, the gauze screen is pretty poor.
Edit, that's a fair point about fouled plugs, mine was doing that when the choke cable seized with the enrichment plunger slightly open and a little fouling coosed it proper. They are a pest to get to but worth pulling and checking.
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