RE: OS 55AX
well.. i got some of my engines running very lean for a error (wrong land adjust) a never had this type of problem.. for me, have more problems there..
but, about cleaning method.. i use pure alcohol after last flight with toothbrush.. if you never allow the castor dry in the engine fins, it´s never will create the difficult to remove "varnish" marks..
BTW, the "Easy of Bang" home clean product is very good to remove a old varnish, but you need leave the engine in this product for a day (24h) to take the varnish more soft and then easy to remove.. it´s works good for me and is very less toxic than antifreezing..
cheers,
Matheus