I also have an mc-18 and it has always run rich at idle. I have 35 pilot jet,Air screw is set at 1 3/4, 152 main on left and 150 on the right. My air box top has been cut open. (I would use caution running your bike with no air box and stock jetting). 35 pilot jet is the smallest I could find. I would try new needle valves and check the fuel tap and see it gas leaks out when it's turned off. Before I replaced the fuel tap on my bike if it sat for a while it would flood the bottom cylinder(and only run on one cylinder). I would have to pull the plug and kick it over a few times to clear it out.
Steve