Pull the plenum off and make sure both pistons are moving full stroke without
any
binding; bit of dust may of found its way onto one of the piston bores.; should
move freely and full with a final clunk.
What altitude are you operating at? Does the vibration alter over 5000'
variance?
Next time the vibration happens activate the choke at very small increments from
neutral position and see if the vibration alters any.
Does the engine idle quality/smoothness also alter?
Sounds like you are running avgas? Best plug colour is double shot late coffee;
light grey indicates lean; light grey yellow indicates water ingress... the
difference
in plug colour between left right bank should be very marginal if not
any. Unleaded fuel should read tad sooty but clean.
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