The twin Mikunis were part of a performance package. They require their own cam and I've got a feeling the distributor was sprung differently as well. Even the secondary vac actuators on the carbs were sprung differently too. I would start by using NGK's or Denso plugs with a higher heat range (maybe disconnecting the vac advance and capping it off...?) I think you're going to need your distributor modified so the engine will have a better tune.