i have a street avenger 670 and the manual said to adjust the A/F ratio using the tach for highest RPMs. i would adjust the screw alot and didnt get a very noticible change. the thing that did change was my gas mileage went down, acceleration went down and spark plugs are wet and black when i check them. i just want to be back where i was atleast! how do i properly set this and where should i set it? i have the following on my engine:

Crower 218/226 @.050 .470/.476
10:1
383 with 71 LT1 stock heads
Bosch Platinum plugs gapped at .050 (summit tech advised gap)
MSD-6A
K & N
holley 7.5psi 60 gph fuel pump with no regulator
MSD super conductor 8.8mm wires
stock HEI

also, which way do you turn the screw to lean the mixture?

 

that's a small carb for a 383.
You may have to drill a couple of idle-air holes in the butterflies so they will close more at idle. That way the transition slots aren't uncovered and feeding extra fuel in at idle.

 

DO NOT! drill the butterflies. This is only done on a motor with a big racing cam.

Adjust the idle speed screw for 650 rpm. Adjust the idle mixture screws to get maximum manifold vacuum. Turn them clockwise to
lean out the mix. Re-adjust the idle speed.
The motor must be fully warmed up to adjust the idle mixture screws.

 

If you wan't them to be back to where they were, just turn the screw in clockwise till it bottoms out, then turn it out 1 1/2 turns. That's how were were most likely were before you touched them. Oh and make sure you do the same to both (one on each side).