[Spridgets] Why Does Engine Like to Run Rich?

[Doug Altman]

Stock 948 with good rings, stock rebuilt H-1 1 1/8" SU's.  I've adjusted the
carbs per the workshop manual, used Colortune, and even Frank's trick of a
standard 2 1/2 turns down (15 flats) on the jet adjusting nuts several times
over the past couple of years and they all point to running a lot leaner than
where the engine runs best.  The engine starts, runs, and accelerates best at
25 flats down (over 4 turns) on both carbs.  Right now it seems to accelerate
best when the front carb is even at 30 flats (5 turns) down.  I've tried rich
needles but it runs for crap with these.  Spark plugs are always black and
sooty.  Why does it like rich?