I have a Holley 390 and 4 bbl carb. it was idling @ 2000 rpms before I changed to headers. now bouncing between 1500-1000 rpm. it will not go any lower the springs aren't pulling hard enough. what to do because I know its burning gas it doesn't need to.

If you have a Holley, I would suggest you buy the Dave Emmanuel book and also the Urich book if available...these are going to be your bible with lots of pictures that can help you tune the carb very well....

If your primary throttle plates are set and they are "shut" under spring tension, there is air being drawn in from somewhere (carb to manifold gasket, manifold gasket)...or your secondary throttle plates are not shut as far as you think they are (this will require you to unbolt the carb, dump it upsidedown over a bucket to catch the gas (use in lawn mower later), and adjust the little screw that is the stop for the secondary plates and vacuum actuator (it's a small technical/eyeglasses style screwdriver)... that will change things....

A couple of exceptions to the rule here:

1) the stock EPA sticker in idle and timing is out the window once you change the compression, cam, or carb setup.
2) A bigger cam has it's power band at a high rpm, if the cam is really lopey or has a real high duration, it will need more rpm at idle to be happy and keep your carb from waffling back on forth between the idle circuit and the power circuit as the vacuum dips into the low numbers then high numbers....
3) You aren't wasting much gas in the idle circuit...800 rpm vs. 1200 rpm really doesn't waste as much gas as expected as long as the carb is still in the idle circuit....the idle circuit is like a squirt gun compared to the main circuit which is a garden hose or the power/enrichment circuit smaller fire hose.... the idle circuit is fairly lean and unless you are the worst tuner in the world the cam/build might be happier at a slightly higher rpm....that being said...if you are idling at 1800 rpm and you are past the transition circuit and into the mains you are burning GPM not getting MPG....


Good Luck, post back results...

Fellow Holley 390 fiddler. Second all of the above advice from DI.

I found that the base of my 390 was distorted, cupped fro PO over tightening it to intake manifold. No amount of spraying around this gasket area would expose this massive vacuum leak.

Remove carburetor, place a know straight edge (I use this) on base in four orientations; N/S, E/W, NE/SW, and NW/ SE using a light looking for any gap between straightedge and base.

If a gap is discovered the fix is to flat file with #12 mill bastard file, which won't deflect, in the above pattern until no light can be seen under straight edge. This can be a tedious irksome task, but it works.

Have a wire brush handy to remove aluminum from file's teeth.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Would be much easier on a flat surface with sand paper . Table saw top or other flat surface .