[chiphead]

I agree, any carb that sits should have a full rebuild.

But if he says its always run like this, it's a tuning issue. The Idle Feed Restriction and the Idle Air bleeds controls the fuel mixture below 2500 RPM. A Holley main circuit doesn't deliver fuel from the boosters until the engine is well into cruise speed. Thats what I didn't realize when I started tuning my 750 DP. Most guys start reducing the size of the main jets, and it's the wrong thing to adjust. The idle feed restriction (IFR) is a smaller jet located in the metering block, that controls the amount of fuel delivered to the transition slots and the idle mixture screws. The transitions are at/above the throttle plates, and the mixture screws feed discharge ports below the throttle plates.

The primary transition slot opening at idle is the absolutely most important thing in a Holley. It gets overlooked, but I now understand it sets the table for the whole fuel map. You want about .015-.030" of the slot showing when the primary blades are on the curb idle screw. At idle, the transition slot bleeds air and fuel around the throttle blades.

As the blades are opened, the transition slot is more exposed. The high speed air around the blades pulls fuel from the slots. The fuel starts to come out of the transition slot more and the idle screw discharge ports less. At 1400-2000 RPM, 90% of the fuel the engine gets is coming from the transition slots. As you open the throttle more, the transition slot gets fully exposed. At that point, the fuel from the transition slot tapers off, and the main boosters start to deliver fuel.

So if you have the transition slot too exposed at idle, you'll get way too much fuel. Then as you accelerate from a stop, you run out of transition slot before the mains come on, so you get a lean spot or a bog or a hesitation. The accel pump can't cover that lean hole up, so the carb runs inconsistent. Rich at idle, and then maybe lean as it transitions to the boosters. Idle screws don't fix it, main jets don't fix it, accel pump doesn't fix it, lots of guys chase their tail.

Since the transition slot is fed from above the idle screws, the screws dont have any affect on fuel available at the slots. The idle mixture screws also become less effective, since the vacuum at the screw discharge ports has started to taper off, in favor of the transition slots.

The correct way to fix this is to close the primary throttle plates until you get about .020" of the slot showing. Then open the idle mixture screws about 2 turns each. There is a small screw underneath the baseplate that lets you adjust secondary idle. Back the screw off and then turn it in, until it just contacts the secondary arm. Then add 1/2 to 5/8 of a turn more, to open the secondary plates just a bit. Stock spec is 1/2 turn.

Fire the engine up and warm to operating temp. Set floats, choke, etc. Get engine to curb idle. Set the idle mixture screws for maximum manifold vacuum. Turn them in 1/8 turn at a time and look for stable vacuum. When the vacuum drops off, you can open the screws 1/8 of a turn and confirm the vacuum comes back up.

If it doesn't want to idle, you can add 2-3 degrees of base timing, that will increase the manifold vacuum and help things. You can also open the primary throttle screw no more than about 1/4 turn, otherwise you'll be exposing too much of the slot. You can also add an additional 1/8 turn to the secondary screw. The secondary screw is very touchy. If it idles too fast, you can reduce the primary/secondary idle speed screws a touch. The trick is to balance the front and rear barrels without either exposing too much/too little primary slot, or opening/closing the secondaries too much. Once it will idle pretty well, then you can set the accel pump so it doesn't hesitate on quick throttle movements.

If it still runs too rich (or too lean) after balancing the transition slot and adjusting the idle mixture screws, that's when you need to go into the primary metering block and look at the sizing of the IFRs to see what is going on. An AFR gauge is really helpful, as I learned. Sometimes what you think the carb is doing, and what its actually doing are very different, and the AFR gauge keeps you sane.

What is the carb list number anyways?