[Tom Vaught]

Good Monday Morning.

Before trying to fix a carburetor issue, it is wise to identify the real cause of the problem.
Carburetors act the way they do for a REASON

Holley Carburetors are not simple devices. Their job is to deliver the correct amount of fuel/air mixture for any given position of the throttle blades. Carburetors do get dirty over time and will require cleaning of the air bleeds with carb cleaner. (WEAR GOGGLES).
You also need to inspect the carb for tight Linkage Arms and use the minimum amount of spring pressure to keep the throttle blades shut. Multiple lighter weight springs is a good idea vs one Heavy "return" spring.

Carburetor problems generally fall into three areas: rich mixture, lean mixture, and incorrect Carburetor(s) adjustment.
Diagnosing carburetor problems seems relatively easy and is usually the first engine component blamed.
Check timing FIRST. Check for Vacuum leaks in the hose connections, check for hoses left off, check for bad ignition wires
Run the engine in the dark and if you have a "Light Show" around the spark plug wires, fix those first before moving to the Holley carburetor.

Actual Carburetor Problems

1) A Rich Mixture means the Fuel/Air Ratio is wrong at some part of the Calibration Curve for the Holley carb.
Common symptoms of a rich Fuel/Air mixture are:

Poor fuel economy
Sluggish acceleration
Choke System not required for "Cold Starts" of the engine
Sooty or black spark plugs
Sooty or black Tail Pipes
Strong smell of gasoline when the Engine is idling
Rising/falling Idle (Engine will typically slow from assumed idle rpm and eventually stall)

2) Lean Mixtures means the carburetor is delivering less fuel than required or too much air. (Possible Vacuum Leaks)
Typical symptoms of a lean mixture are:

Backfires as the throttle is closed (primarily during coast-downs). (A Coastdown is when the engine is under no load)
Surging/ missing on acceleration (Lean Back-fires)
White or light gray spark plugs
Engine runs only when the choke is partially closed
White or light gray exhaust pipe tip openings Not as common with today's modern fuels vs 1960s fuels

3) Carburetors not syncronized refers to carburetors that have an incorrect adjustment of the fuel/air ratio and the balance between two or more carburetors is poor.

Poor overall performance
Engine tends to stall easily, have to keep it reved up. Tri-Power Carbs with Blade to Bore settings wrong suffer from this.
(some like myself, do run multiple Holley carbs on engines occasionally)
Poor acceleration
Poor fuel economy
Misfires and/or backfires

Correcting Carburetor Problems

Lean Mixtures: This condition is usually caused by installation of after-market parts not calibrated for the engine parameters:

Poorly designed air filter systems for the Holley Bowl Vent Systems, or replacement carburetors of a different type or size.
Fuel level in the Fuel Bowl is set too low, this will change the calibration in the Emulsion Circuit and insufficient fuel will be drawn through the main jet

If no changes have been made to the engine, and it previously ran fine, a lean mixture can sometimes be caused by a leak at the intake manifold or from a leak at a exhaust gasket.

Rich Mixtures: This condition is typically caused by a dirty air filter, but it could also be caused by a poor selection of a replacement exhaust system (muffler) and/or carburetor system (Dominator carb on stock engine).

If the fuel level is set too high in the fuel Bowl, a rich mixture can result.

Idle Eze Screw. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will, therefore, richen the mixture but now you may not have sufficient airflow to keep the engine running at the assumed idle speed.

Incorrect Carburetor Adjustment: Many times, more experience is required or help from a friend needed. This is rarely caused by poor maintenance. More like 'over maintenance'. The enthusiast loses the 'Tune-up' and does not know how to get it back so he buys a different carb. If he reset the original carb back to factory specs the carb issue would very likely go away.

Tom V.