I have the exact same problem with my OS .46 FX, a colder plug made it worse, tried a different fuel, different muffler,even different props, replaced all lines including the klunk,(yes I reset my high and low mixtures with every change, any leaner on low end resulted in too lean). This was all last season. Put a new piston and liner in it last week and broke it in on a 4x4 clamped to a sawhorse, ran great throughout the RPM range. When I screwed it to my Ultra Stick, my problem was back. The tank is centered with the spraybar. I've noticed that the engine seems to blur (visually) at the same rpm that it seems to load up at. I now suspect that the Dave Brown mount I'm using on the Ultra Stik is maybe too flexible and the excessive vibration at midrange is somehow affecting the mixture. The break-in mount had a sturdy aluminum mount, and it ran great there the day before. I'm going to try a different mount. I'll post the results.

JJ