All my four strokes are mounted inverted without a single starting or idling problem. Each one took some time to get the low end needle dialed in, however. I basically leaned out the low end until the idle speed only dropped slightly when the glow plug igniter was removed. I also made sure the engine can idle for a couple of minutes and doesn't quit when I abruptly advance to full throttle. I don't touch the low end ever again once the "sweet spot" was found. The 2 cycle engine will be more tricky to tune the low end. I had a 2 cycle engine mounted inverted once a while back, and it was a real pain to tune the low end and to get it to start.