road grader #12 black smoke low power

Sounds like it’s getting choked for air to me. Check all the inlet pipes for obstruction like a mud dauber nest etc. then verify the turbo impeller is really turning and not bound up. Your fluctuations on the gauge might be interpreted as a valve sticking. Obviously it’s getting plenty of fuel so I’d rule that out. Might also take the muffler off, if it has one, that would rule out exhaust restrictions.

Let us know how you come out.

BP.

Removing the oil cup will not make much difference in the restriction caused by the air cleaner. The screens that can be removed and the "steel wool" packing above them may be packed solid with dust and oil that has turned into asphalt.

The recommended cleaning procedure is to immerse the whole canister in solvent like mineral spirits, kerosene or diesel and plumb an air line to bubble up under it so there is agitation, turn on the bubbler and let it go for some hours. Steam cleaning and other water washing procedures can leave any remaining dust and oil not removed in an even more impervious state than before you started. So if you go that route, get it CLEAN!

Someone would have to have recalibrated the governor or changed pumps, nozzles etc to have a 1950's #12grader that required turbo boost to NOT smoke. The CAT turbos of that era were used only to "normalize" the engine performance at high altitudes

The 12E grader with the D333 introduced in1960 still did not use the turbo boost to increase horsepower. Prior to that there were D318 (G?) turbocharged engines I think were used in #14 graders that utilized the turbo along with higher fuel rate for increased horsepower as well as industrial power units of the same era similarly equipped, so it's possible the engine might be a transplant.

One thing nobody has asked is if it has excessive blowby coming out of the crankcase breather. Especially if that has a "huffing" quality to it, I would suspect one or more damaged pistons possibly resulting from stuck rings.

Thanks everyone for the suggestions. I will follow up on those and get back with the results.

ccjersey, I can't find an engine model number, just the serial number #8T19245. Is that enough to ID the engine? As far as I know it is the original engine, and it exactly matches the pictures in the manual. We bought it from the County, and a friend who worked at the county shop for many years said that they had it rebuilt, but doesn't recall it ever being replaced. they also used to have another one supposedly like this one except it didn't have the turbo. We are at 3000 ft, so maybe they ordered this one with the turbo for that reason, as you suggest.

The air pipes to and from the turbo are several cast sections. Are they just a friction fit between sections or is there some kind of sealer? Any particular cautions in separating them?

Thanks again. I will report back.
Bob

The air pipe joints are either gasketed or use o-ring type internal seals.

I am surprised that it still smokes with no restriction to the intake, but you did say it smoked badly even when it had 1 or 2 psi boost, so I guess that is to be expected.

The grader serial number on the engine tag does indicate it is original to the machine.....though it could have had the tag moved if the engine were ever replaced. But you have a history that it is original so I would go with that. 8T19245 was manufactured in 1956

The #12 graders manufactured from 1948 to 1959 all used the 4.5" bore 6 cylinder D318 engine with the governor set at a higher rpm and probably increased rack setting compared to the D6 tractors I am used to dealing with. It is likely that the fuel delivery of that engine is increased beyond the normal non-turbo grader and the turbo is required to make it perform.

The only other thing I can think of is if the engine was trying to lock up and spin a bearing or the clutch was somehow dragging, the governor might go to full fuel attempting to bring the rpm up and cause it to smoke badly.