The only reason I am scared to run the .031 idle feed restrictors is because last time I had them in the carb 4 of my plugs fouled at idle and I was setting my idle mixture screws to the best vaccum reading in drive with the brake applied. That didn't matter plugs stil fouled.
I have noticed my engine fouls plugs super easy, and I did take care of the vent, I didn't break the vent I just simpily forgot to put it back in.
You also have to remember I live 4000 feet above sea level, so since .031's work fine on your 340 right now, if you came up here you might notice they are too big. I don't want to foul any more plugs, I am tired of removing wires and plugs all the time. I have 8 fresh plugs in there right now and I am about 80 percent sure if I changed back to the .031 idle feed restrictors some of my plugs would be fouled regardless of where my idle mixture screws were. And I just double checked my idle mixture screws and they are all out about 1 turn.