You mean hiccupping I assume. That's why I don't suspect ignition, add to that all plugs being sooty makes me think the ignition was strong...
Basically, from my experience I don't think it's ignition related.
The proper base gasket for the YF is also critical for vacuum.
If I remember correctly the metering rod setting was critical, along with the proper rod actually being available. I finally gave up/took a break...
Plugs were sooty? Running rich to begin with apparently.
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