Now that I have your attention, here is what I am trying to figure out...
My D2 does this: When running at idle, it occasionally puffs blue smoke- just a couple puffs then it is clear again. Then a little bit later, it does it again. Under higher throttle settings it runs perfectly clear all the time- no blue smoke, and does not even get black under heavy load (needs governor adjustment?).
So, I ask myself, why is it doing this? Here are my theories, in order of perceived likeliness.
A. Exhaust valve guides and seals need replacing: Oil pools up on the head and then washes down the guide(s) when it gets high enough; this drains it, and then it slowly pools up until it does it again. At higher rpm the higher pressures keep the oil from running down the guides.
B. Rings are going, but at higher rpm there is enough pressure to keep blowing the oil back down the cylinder walls and/or the rings provide better sealing then. (Would not explain the intermittent nature of the issue- unless it maybe just one cylinder that is bad)
Note that this condition persists after several hours of running with a moderate load on the machine.
History: Grampa and his sidekick Bill pulled the head and had it completely re-done in the 1980's. (I have the receipts). New valves, guides, seals, had it milled, the works. They also pulled the pistons out through the bottom and replaced the rings. Nobody can tell me how much the cylinders looked worn at that time, obviously not bad enough to do a bore job. Machine has probably no more than 1000 hours since then.
One other note: Dad tells me that they ALWAYS have had to use ether to get her to start from cold, before and after the rebuild. My experience has been the same. (I suspect this is a low compression issue, though you would think new rings and valves would have fixed it.)Since I had the lift pump rebuilt (thanks Walt) and put in new FI nozzles and pumps, she seems to run pretty strong- I think. The other day I was pushing over a 10" pine tree and it spun the tracks rather than stalled, for whatever that's worth.
Can't wait to hear your ideas! I really hope that another head rebuild would solve things, as I really do not want to pull the lower end and buy new pistons etc. Machine has excellent oil pressure.
Greg