"Excessive black smoke means the fuel is set too rich. An old Bohunk CAT mechanic told my Dad years ago that when working their hardest, those old machines should be putting a slight haze out of the exhaust. That's how he told if the rack was set right. Your results may differ, though.....
Would altitude figure into this in any way, Silver???"
This tickled me---my dad grew up some in mining towns--'bohunk' 'wops' and 'micks'---and probably others---not as politically uncorrect as todays hypersensitive society might deem, perhaps more of a description/slang
anyhow, yrs ago i had a 335 rebuilt in great falls by a great ol' "bohunk"--this guy started out doing pumps and injectors---then expanded into a few complete overhauls----he'd do some stuff to cummins specs, considered other specs plumb sloppy---i bought this motor well used, it had 380 stamped on the pump but he set everything for a 335. Guys we were running with saw the 380 and said "a ha! knew that had more than a 335 in it!" It'd avg 8 mpg, and that's counting woods roads and loading yourself. And with self loader and pusher axle I was was at least 6,000 # heavier that them.
anyhow, "Babe" said 'turning up a pump' was bull---that pump was just one component and that injectors, turbo and pump should be matched to each other and a planned design on the rest of the engine. Very sweet, honest engine. On a long pull, just the slightest hint of black smoke. "That's unburnt fuel---don't do anybody or anything any good"
I'm wondering if your engines turboed? Or set for your altitude? And what kinda shape your injectors are in? And any restrictions in fuel, air supply? I love old cats---my D7 is a 7M--precedes the 3T---made early 40's---wide open is 'high idle'---turns slow and overbuilt, can't tear itself up---but it will slip tracks in 1st.