I would try Techron. I've used it over the years with visible results. Couple carbed motorcycles mainly. Looked behind the valves before and after with stunning results. On my 1998 Kawasaki ZX9R, the valves were loaded with carbon chunks. One tank later about 98% of the carbon was gone. Also Porsche recommends it. Confident they know their sh_t. You were talking varnish tough. I would try Techron or B-12. Both are solid products.But what would be the best carb cleaner to put in your gas, even if its still a little varnishy after the ultrasonic cleaner?