Taper bore cylinders need to get hot in order for the parts to have the right clearance. If you run 4-cycle rich during break-in, you are allowing continued interference fit by not letting the cylincer expand enough from being too cool. That will cause premature wear at best, and as 2slow mentioned, could result in worse. I would not recommend that on a taper fit bore. run it rich, but not 4-cycle rich.