What would cause my fz91 to go dead rich after 20 or 30 seconds at idle? It runs great at midrange and above, and for a few seconds at idle, but then goes very rich and takes forever to clean up when the throttle is opened. I've set the idle as lean as posible (yes, I understand that the screw works oposite what we're used to) and turned the regulator in some, all to no avail.

I'm using Mach 7 15%, castor/synthetic blend, OS Type F plug, APC 14X8 or 14X10 prop.

It's been suggested that I need more nitro, or eliminate the castor, but this seems more like an adjustment or leakage problem to me since it runs so strong otherwise....