Check your electrical before building carbs. Like said before check around your control box for bad wire or loose connections. Take cowl of and have someone move throttle back and forth while sitting in drive way. Look at wiring coming out from under flywheel. Your timing advance is manual and there is some wiring close to the advance mechanism that could be rubbed. Especially check wiring and switch for kill switch. Your bouncing in the seat is moving something, throttle control box or steering wheel.
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Charlie Weaver USN/ENC 1965-1979