Rich and lean ....

Hi all . another Question for you clever people ..

I am running a 600 Holley and doing the final settings and I cant for the life of me remember ... is it screw the air screws in to richen the mixture and screw out to weaken ?

Thanks guys John..
Depends on the model you have. The early or "traditional" Holleys lean as the screw is turned in (cuts off fuel when screwed all the way in). The later emissions had a nylon thingy on the screw, and they were opposite. Long story short:

There are two different types of idle screw circuits used in our Holley carbs.

The regular type we are aware of, are FUEL screws, in leans out, out richens. These screws have the same outer diameter their entire length.
The "reverse" type is an AIR screw, in richens, out leans, to a point. These screws have an enlarged head with serrations at the outer end of the screw, and are usually covered by a plastic limiter cap over the end of the screw.


Gold Supporter
I first set the carb level when the car is level using a wedge plate. As my engine is slightly nose down and the Edelbrock manifold is angled the other way i use a 5 degree wedge plate to get the carb level. My Holley is the type where the mixture screws adjust the fuel flow and hence in is leaner and out is richer. Set the fuel pressure to 6psi.
I then set the fuel levels on both sides by adjusting the float screw. With the fuel pump running to maintain fuel pressure remove the sight level screw. Have a small container or a rag ready to catch the fuel if the fuel level is way too high and a lot runs out. Loosen the big slot screw and the adjust the float level nut until the fuel only just weeps from the sight hole. Then retighten the big slot screw and replace the sight plug. Be careful not to damage the very thin washers on the level nuts. They are very thin so I have some spare just in case I mash them.
Turn the mixture screws fully in but only very lightly and then unscrew 1.5 turns. This should be near enough to get it running well enough to get the engine warmed up. Then adjust both the mixture screws 1/8th of a turn in or out until the best idle is achieved. Or attach a vacuum gauge to the inlet manifold and adjust for maximum vacuum.


jac mac

New Member
With regard to setting the carb pad level, take a closer look at your Holley and you will find the fuel level sight screw plugs are slightly lower on front and higher on the rear float bowl to allow for the nose high engine attitude on virtually every production car they are fitted to.