Idle bar plugs began when engines started to be throttled and it was soon found out that idling was a problem, mainly because of the too simple carbs that couldn't regulate the idle mix very well. There weren't any mufflers back then either so exhaust baffles were used to help keep some heat inside the cylinder. The idle bar helped to stop any raw drops of fuel which collected in the crankcase at idle (because of the pathetic carbs ) from putting out the fire. Enya overcame the idle mix problem by introducing a carb that had twin needles where one needle tuned the idle mix and the other tuned the high speed but they had to revert to the earlier type carb because modellers thought the twin needle carb to be too complicated so didn't like it. It wasn't until about 10 years later that twin needle carbs began to appear again. Sometimes it doesn't pay to be ahead of the game .

The only engines where you have to be really careful about plug reach is on some of the older (non Schneurle) baffled piston engines. A lot of them had the plug off set to the transfer port side and the plug was located directly above the baffle. Too long a plug would then extend down into the baffle slot in the head and physical contact would be made. Not good . An idle bar plug would only make things even worse. With modern Schneurle port engines this can't happen unless you have a very small engine. My smallest is an OS .10 and I could only get slight contact by using a long reach idle bar plug without the washer fitted.

I must admit I'm surprised that the turbo plugs (as used by a lot of car engines) haven't started to appear on plane engines because they avoid problems with plug length, give a better combustion chamber shape and are now available in a quite a few heat ranges.