Race....idlebar plugs came about by Duke Fox in an attempt to keep his crummy
carbs from throwing droplets of fuel into the combustion chamber and drowning
the plug. They were not invented because of loop scavenged engines like the K&B's
Fox didn't invent them. I think Champion invented them. K&B has their own idle bar plugs, you needed them on at least some of their baffled engines not the loop scavenged ones. I think MDS is the only looped scavanged engines that need them. I had to use idle bar plugs on my first R/C engine a baffled Supertigre Blue Head .60. That engine had a fine carb. As did many Fox's especially the MK series carb, almost a copy of a Supertigre carb with crude needles. My Fox .50 runs great with an OS #8 plug and 5% fuel.