Jan 06
Having owned about seven OHV sixes, I found they start to get smokey
right around 40,000 miles. The late '62s and onward, with the full flow
filter were a little better, but those would get smokey around 50,000
miles.  I found the drain-back holes behind the oil rings get packed
full of hard carbon.  The carbon is so hard that it takes a drill bit,
to clean them out.  If you drive them when they burn oil (200-400 miles
per quart), the excess carbon also builds up on the valve faces causing
them to burn.  Burned valves, then leads to the famous cracked head
problem, which is pretty well known on this engine.