I beleive the 2+4 system (still
) used on the 115 mercs works by restricting the fuel through carbs #3 and #4 at idle and up to 2000rpm. When the engine reaches 2000rpm an accelerator pump dumps a load of fuel in the 2 carbs and from there on the carbs run normally. There are no setable idle jets on carbs #3 or #4 and the accelerator has 2 special outlets for these 2 carbs. I have never heard of a switch in the ignition system cutting out 2 cylinders at idle.
Maybe the conversion involves removing carbs #3 and #4 and the accellerator outlets to these 2 carbs and replacing them with conventional carbs like #1 and #2?