I have a Speed Demon 750 w/ vacuum secondaries sitting on my ZZ430 clone. (Thats essentially a ZZ-4 with Fast Burn aluminum heads and the GM "Hot Cam" and 1.6 rockers. It's running really fat. The plugs are sooty black, and so are the tailpipes.

I've very carefully set the butterflies, the floats are right on the money, the fuel pressure is about 6 lbs. Idle mixture screws have been adjusted and re-adjusted, and are about 1 turn out. Power valve is 6.5, and the engine pulls a 12" of vacuum at idle. I have a standard Chevy 350 PCV valve in one cover, and a vent in the other.

Timing is per spec - 10 initial and a max of 32 at about 3,000 RPM. I'm running manifold vacuum to the distributor, but have it limited to 10* max, per Sallee Chevrolet recommendation, using a Crane adjustable vacuum control. This engine does not like a lot of advance.

The jets that came with the carb were 83 secondary and 76 primary. I dropped down two sizes on all four corners 81 & 74. Still looks pretty sooty.

I have a tough time putting in fresh plugs and trying to read them because I live in the middle of a subdivision.

Any thoughts?