I would see what the squirt looks like in your driveway at the offending RPM that it does it out on the road. You wouldn't be under a load but it might show something, it has to be lean for a reason & you'd think it'd be a reasonably easy thing to pin down. my 1406 needed richening (rods/jets) & would bog cuz the choke came off to quick (& I'm talking normal driving