I've found that an engine will idle bad if.. its too tight (not fully broken in yet), too loose which doesn't sound like your case, too rich or too lean (fuel coming out the exhaust?), or like said, a bad plug
Have you tried letting the engine fully warm up before taking the glow heater off? that any better?
My picco 28, I have to let it get hot before I pull the heater off.. but its pig rich and virgin tight
