I had many of the same problems last year with my 250. It was also a 2 carb setup, had good compression, very high oil consumption but no smoke. It also had a rattle in #5 & #6. Cure: total rebuild, I galled the pistons due to a lean condion on rear carb. After the total rebuild I replaced the two carbs with one 390 cfm Holley 4bbl and a Offy intake. It now starts, idles, and runs great except for a flat spot off-idle which I haven't had time to address.
