How long to you try to let the engine idle at? One doesn't really need to have long engine idle times.
I have a couple of .61 four strokes that didn't work right until I used 15% to 20% nitromethane in the glow fuel. Then those engines ran good. Before with 5% nitro glow fuel they didn't idle well at all. usually I don't have problems with low nitro or no nitro glow fuel in my engines, but I have a few that want more nitro in the fuel.