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Carb Theory and Diagnosis

It is a subject of great amusement to my friends and colleagues that I know practically nothing about electricity, other than throwing a toaster into the hot-tub is a very bad idea. From a science perspective  I really don't know why it's a bad idea - other than it makes you dead.

I do know a bit about the increasingly lost art of carburetion.  Over the past 30 years, we've been asked to sort out many foul running vehicles and most of them had carbs that were bodged beyond hope.  Carburetors are very precise metering devices, and the most minuscule error in adjustment or jet size can bring an otherwise perfect engine to it's knees.  Details and knowledge count here!

Anyone under age 30 can turn the page now.  You've never seen a carburetor, vinyl record or rotary dial phone. Laugh all you want, and go back to your X-Box.  Just remember one day you'll be old too - and the young guys with their 10 gigawatt flux capacitors will be laughing at your antique Subaru WRX. Us old farts should read along.

This article illustrates a highly simplified view of how a basic carburetor works and common problems and troubleshooting hints. You won't find tips on the nuances of the various carbs on the market, but tuning of any carb first starts with an iron-clad understanding of carburetor basics.  The concepts are simple -  especially if you think about the basic principles of vacuum that are involved.

We hope you find this article helpful. Email us with your thoughts, tech requests and feedback at  We've also written a pretty thorough guide to tuning the Carter AFB and Edlebrock Performer series.  Let us know if you'd like us to publish it here.

A carb has 3 fuel circuits - idle, primary and enrichment.  The idle circuit handles carb operation from idle speed to approximately 1/8 throttle (particularly for the Carter AFB and Edlebrock Performer Series) The primary circuit governs operation from 1/8 throttle through wide open throttle.  The enrichment circuit uses an accelerator pump to provide short-term, additional fuel beyond what is supplied by the primary circuit.  This is needed whenever you sharply whack the throttle directly off idle or from a steady state cruise.  Lastly, there is the cold operation choke mechanism.  This is not a fuel circuit per-se - it uses a mechanically actuated choke plate to alter how vacuum and air/fuel act upon the existing idle and primary metering circuits.

Sounds complicated, but it's really not that bad.  Let's describe 4 basic operating scenarios

Warm Idle:

Let's start with the idle circuit on a warm engine.  We'll cover the cold idle choke operation in a moment.  In this scenario, your foot is off the accelerator and the engine is operating in the 500-1,000 RPM range.  The throttle blade is basically closed (though not fully - the engine requires some air to support combustion)  The closed throttle blade creates a very strong vacuum signal under the blade as the piston moves down the cylinder on the intake stroke - and this vacuum pulls fuel out of the idle slot, located just below the throttle blade.

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

When things go wrong with the idle circuit, don't immediately start turning wrenches.  Take a systematic approach to diagnosing and eliminating problems - realizing that most idle problems are related to incorrect idle screw adjustments, idle mixture screw adjustments and vacuum leaks.  The idle adjustment generally involves setting base idle speed with the idle speed screw, adjusting the idle mixture screws until the highest idle speed or manifold vacuum is attained and then bringing the idle to the factory spec using minor tweaks to the idle speed screw.

If idle speed is not at all impacted by adjustments to the idle mixture screw(s) then odds are very good you've got a vacuum leak. In order of commonality the sources of those leaks are; loose/disconnected vacuum hoses, loose carb mounting bolts, bad intake gaskets, leaky headlamp/climate control reservoirs, defective brake boosters. Any vacuum leak (no matter how small you believe it is) is a bad thing.   No amount of carb fiddling will help until you fix it!

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

Cold Start:

Now, let's describe the cold start situation - which applies whenever the engine has been shut down for more than a few hours.  In this situation, the intake manifold is cold, and atomized air and fuel entering the runners quickly condenses and the fuel "falls out" of suspension in the air.  This fuel wets down the inside of the intake runners - and not enough reaches the combustion chambers to reliably light off the engine.

Cold start involves the choke mechanism and gets a little more complicated.  The key to understanding the choke operation is that it is a thermally operated device.  When the engine is cold the choke plate closes and vice versa. A closed choke plate shifts the engine vacuum signal higher up in the carb venturi so that the booster is exposed to unusually  strong vacuum forces.  This pulls a lot of atomized fuel and air into the intake manifold to help the engine start and run as it warms up.

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

There are a few things that can go wrong with the choke mechanism.  Most commonly, the actuating spring is out of adjustment and fails to close or open the choke plate adequately.  Also, the choke spring can be installed 180 degrees backwards, so that the choke plate opens when it should close (and vice versa)  Another common problem is that the intake manifold has been swapped for an aftermarket unit - and the runner size, shape or length has changed - which impacts how effectively the cold atomized fuel/air mixture reaches the combustion chamber.  Lastly, some aftermarket intakes have no provision for exhaust heat crossover or are "air gap" designs - so the intake runners stay cold for far longer than an OEM unit.

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

Gentle Part Throttle:

"Gentle part throttle" describes the condition when you pull away from the curb and apply a very gradual and light pressure to the accelerator pedal - e.g., when your mother-in-law is riding along on the way to church.  In this scenario, there is a orderly transition of command from the idle circuit over to the primary metering circuit as the throttle blade(s) open.  By the time you reach 1/8 throttle or so, the primary metering circuit is fully operational and the idle circuit has shut down.  The key to understanding this transition is to understand how air and vacuum flowing past the booster changes as the throttle opens.  This is critical.  Note that in this illustration we've taken away the choke mechanism to simplify the description.

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

When the primary metering circuit goes wrong, you may experience a range of symptoms.  This may include; a surge in vehicle cruising speed, a bog, or black smoke from the tailpipes.  Generally,  a surge means a lean mixture (the engine isn't getting enough fuel) while a bog/black smoke means a rich mixture (the engine is getting too much fuel)  As you can see from the illustration below, there are more parts involved with the primary metering circuit - and correcting these can require a dismantling of the carb.  Note that if you've got a GM Quadrajet, Carter AFB or Edlebrock Performer, the primary metering circuit also includes metering rods and springs - but this is more complexity than we can cover here.  The basic principle remains the same however.

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

Hard Throttle:

OK, now we're having fun.  You pull out of the Burger King parking lot, and whack the throttle - ready to lay down a glorious ribbon of burned rubber.  And, your car stumbles and stalls.  Your girlfriend hates you and everyone thinks you're a nerd.  Avoid this tragic outcome by knowing the enrichment circuit.  It is your friend, and exists to help make you look cool. It is the key to a really stellar burnout.  Take time to get to know it.

When the throttle blade opens super quick, there is a slight time lag before the vacuum signal at the booster can begin to pull adequate fuel to support the engine's demands.  All enrichment circuits involve a carb linkage actuated accelerator pump, and when you whack the throttle sharply - it blasts a little shot of extra fuel down the carb throat.  This squirt of fuel from the accelerator pump helps to bridge that timing gap until the booster can begin to flow.  The amount of fuel required (and the duration of the fuel shot) are a complex function of carb size, booster design (annular booster carbs like the Autolite 2100 and 4100 have the best booster design - it doesn't take much vacuum to make an annular booster flow a nice atomized mixture of air and fuel) vehicle weight, gearing, converter spec.  Getting it right involves some trial and error.


Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

Depending on the carb you've got - the process for correcting enrichment circuit problems can be easy or a major league pain.  Holley does have provision for fine tuning both the amount of enrichment fuel, and the exact timing and duration of when it is supplied to the engine.  Lots of folks love Holley carbs, but I think their enrichment circuit design is overkill for most users - way too complex and too easy to screw up.  Carter, Edlebrock, Quadrajet and Autolite carb enrichment circuits are simpler and much easier to correct, involving little more that repositioning the accelerator pump linkage to alter the timing and volume of the pump shot.

Illustration © Copyright Gyrhead & Sons Restoration Parts 2012

So there you have it.  Carb operation explained in eight simple pictures.  We hope this guide helps you to become an expert on the disappearing art of carburetor tuning.  Please let us know what you think.

© Gyrhead & Sons Restoration Parts 2012.  If you like this article, feel free to share it with your friends.  Just remember to cite the source, or else everyone will think you're a big jerk

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