Under the Hood: Counting Carbs
By Larry Carley
Dissecting Old School Performance Fuel Systems
Carburetors have been around forever, and are still the dominant fuel mixer for most circle track, drag racing and street performance muscle cars.
Carburetors persist because they are relatively simple and inexpensive compared to electronic fuel injection (EFI). Most racers and hot rodders are familiar with carburetors and know something about how to install, tune and adjust a carburetor.
They may not always do it right, but all they usually need is a screwdriver and a wrench. They don’t need a laptop computer, a scan tool or any other fancy electric hardware to play with the fuel settings.
However, as popular as carburetors are for performance engines, they do have some weaknesses and those weaknesses, if not addressed or corrected, can lead to significant issues.
The greatest danger is not jetting a carburetor properly which can cause the engine to run dangerously lean. If an engine leans out under load or at high rpm, it can go into detonation and sometimes burn a piston. That’s the number one cause of fuel-related engine failures.
If a customer’s engine has burned a piston, it’s likely the engine is running out fuel for one reason or another. The underlying cause could be the carburetor is calibrated too lean, there’s a big air leak somewhere that’s allowing air to be sucked directly into the intake manifold or the fuel pump is starving or can’t keep up with the engine’s fuel requirements under hard acceleration or at high rpm.
Whatever the cause is, you’d better help your customer figure it out otherwise he’ll have the same problem again and again.
Most carburetors don’t need much fuel pressure (5 to 8 PSI), so if somebody installs a killer high volume fuel pump without an inline pressure regulator, or they set the regulator too high, there’s a danger of forcing fuel past the needle valve and flooding the carburetor and engine (which can start a fire!).
On the other hand, if they are running a stock fuel pump with a high revving or large displacement engine, the pump may not be capable of delivering enough volume to keep up, which will cause the air/fuel mixture to go lean.
Drag racers often use two fuel pumps to make sure the carb doesn’t starve for fuel under hard acceleration: an electric “pusher” pump mounted in or near the fuel tank, and a second electric or mechanical pump by the engine to feed the carb. Restrictions or kinks in the fuel line, or even a plugged fuel filter (or one that doesn’t flow enough fuel) can interfere with a steady supply of fuel to the engine.
Idle quality is another issue that’s common with carbureted performance engines, especially those with long duration camshafts. A carburetor uses intake vacuum to pull fuel from the fuel bowl(s) through the metering circuits and into the discharge ports. A stock engine with a relatively short duration cam usually idles well (unless it has a vacuum leak somewhere) because it usually has a lot of intake vacuum (18 inches or higher). But a performance engine with a hot cam may only pull 15 inches or less of vacuum at idle. That makes for a hot-sounding lopy idle, but it can cause the engine to hesitate, stumble and stall when the throttle is cracked open.
Idle quality can usually be enhanced by using a carburetor with venturi boosters in the main venturis (the narrow part of the carburetor throat), or slightly smaller venturis. When air passes through the smaller cross-sectional area of the venturi, it speeds up, like air passing over the top of a wing on an airplane. This creates an area of low pressure in the venturi that helps suck fuel through the metering circuits and discharge ports.
Another trick is to mount a four hole spacer under the carburetor. Raising the carburetor an inch or two and forcing the incoming air/fuel mixture to flow down the holes in the spacer also increase air velocity, which helps idle quality, throttle response and low end torque.
The design of the intake manifold also plays a role in idle quality, throttle response and where the engine develops peak power.
If an engine you are building is for the street where low end torque and everyday driveability are essential, the intake manifold should be a split-plane 180 degree style manifold. Hi-rise manifolds with longer runners and taller plenums will maintain greater air velocity to create more low end power and torque.
By comparison, if you’re building a high revving race-only engine, go with an open plenum 360 degree style manifold with large runners, and make sure the plenum and runners are properly sized to match the engine’s displacement (bigger engines need intake manifolds with larger plenums and runners).
Idle quality usually doesn’t matter much in a race car, as long as the engine will idle without stalling. But a street-driven vehicle is an entirely different story. Street engines spend a lot of time idling, so good idle quality is essential for everyday driveability.
As far as the carburetor is concerned, idle quality is affected by two things: idle speed and the idle mixture adjustment screws. Set the idle speed too low and the engine may stall. Set the idle speed too high and it may not work well with an automatic transmission.
The idle mixture screws change the air/fuel ratio at idle only. Once the throttles open past the idle discharge ports in the throttle bores, the engine pulls most of its fuel through the discharge ports in the venturis.
Idle quality (or the lack thereof) can also be affected by air or vacuum leaks that allow air to be pulled into the intake manifold.
The leaks may occur past the throttle shafts if the shafts are worn, or past the carburetor base basket, adapter plate or spacer gaskets, the intake manifold gaskets, or any vacuum or plumbing fittings that connect to the manifold.
Vacuum leaks can often be found by spraying carburetor cleaner or propane vapor around suspected leak points while the engine is idling. If the idle suddenly smooths out, you’ve found a leak that needs to be fixed.
Recarbo, a company that rebuilds carburetors, insists that most used carburetors typically have life left in them and, if rebuilt and set-up correctly, can provide reliable service for a long time. Each carb from this supplier gets tested and run on a live running engine before it goes out, and all the adjustments are set too. In the background of the photo above, you can see the computer screen full of information that the technician is watching throughout the test.
One of the most annoying problems that can occur with a carburetor on a performance engine is a flat spot, stumble or hesitation when you floor it. When the throttles open wide, the engine inhales a big gulp of air. Intake manifold vacuum drops to almost zero, causing the velocity of the air moving through the carburetor to momentarily slow down until it starts to speed back up.
If the accelerator pump on the carburetor doesn’t squirt enough extra fuel out of its discharge nozzles to keep the air/fuel mixture from going lean, the engine will likely cough, stumble and maybe even backfire in protest. So one of the first things that’s usually modified on a performance carburetor is the accelerator pump. The carburetor may be fitted with a larger displacement pump and larger discharge nozzles. The pump linkage can also be modified to make sure the pump delivers an adequate shot of fuel when it’s needed.
Many carburetors also have a vacuum-acutated “power valve” that opens when the engine is under load and intake vacuum drops. The power valve allows more fuel to flow into the engine to help richen the fuel mixture. Like the accelerator pump, it can cause a flat spot or loss of power if it fails to deliver enough fuel when the engine needs it.
A carbureted performance engine that is hard to start may not fire up easily for one of two reasons: either it is getting too much fuel (flooding) or not enough fuel (starving).
Stock carburetors have a choke in the top of the airhorn to restrict airflow when a cold engine is first started. Obstructing the airhorn does two things: it reduces the amount of air that enters the carburetor to richen up the air/fuel mixture so the engine will start easier, and it creates a partial vacuum that helps pull fuel through the metering circuits.
Performance carburetors. Photo courtesy of Holly.
The choke mechanism also has a stepped linkage that holds the throttle a little further open to increase the cold engine’s idle speed. This helps prevent stalling and improves cold driveability.
As the engine warms up, a temperature-sensing coil spring inside the automatic choke housing warms up, expands and gradually opens up the choke plate to allow more air into the engine. The cold idle cam on the throttle linkage also moves to bring the idle speed back down to normal.
Automatic chokes can cause hard starting if the choke housing is not adjusted properly and is set too rich or too lean, or the choke linkage or plate binds and fails to open or close when it should.
If the engine has a manual choke setup, the driver has to remember to pull the choke cable knob out before he starts the engine, then to gradually push the choke cable knob back in as the engine warms up, otherwise the engine will start to run too rich, lope and stall.
Most performance carburetors don’t have chokes because the choke plate restricts airflow. Consequently, a cold race engine may be hard to start on a cold day and require a couple shots of starting fluid (ether) to get it going. The lack of a choke also means the engine may not idle well or hesitate until it warms up.
Beware of Flooding
Flooding happens when the engine gets too much fuel. It’s another problem that can plague a carbureted engine and can happen while trying to start a cold engine.
A cold engine needs some extra fuel to get it going because fuel droplets evaporate slowly and mix poorly with air in the intake manifold when the engine is cold. Once the engine fires up and heat starts to build up, the fuel is vaporized more easily and mixes better with the air. But if a cold engine gets too much fuel while it is being cranked, it may not start at all. The liquid fuel will foul the spark plugs, and may even wash much of the lubrication off the cylinder walls, increasing the risk of piston scuffing.
A couple of light pumps on the gas pedal are usually all that’s needed to start a cold engine. But if somebody has installed a larger accelerator pump and pumps too much, he can easily flood the engine, making it hard to start.
Optimizing the Air/Fuel Ratio
Performance engines run best with an air/fuel mixture in the range of 12 to 13 to 1. This isn’t the cleanest or most efficient mixture, but it typically
delivers the most power.
Emissions-controlled engines, by comparison, are calibrated to run as close to the ideal or “stoichiometric” air/fuel mixture of 14.7 to 1 because this produces the lowest overall emissions. Leaner mixtures are better for fuel economy, up to a point. But beyond 18:1, there’s a risk of lean misfire, a significant loss of power and the risk of detonation/preignition and possibly burning a piston.
Richer mixtures (less than 12 to 1) just waste fuel and don’t deliver any more power because there’s not enough air to burn all of the fuel. And if the mixture is less than about 8:1, it may not ignite at all causing the engine to misfire.
Keeping the fuel mixture within the optimum range is a delicate balancing act. The air/fuel mixture delivered by a carburetor depends on the height of the float setting inside the fuel bowl, the size of the main metering jets, and the size of the air bleeds and/or emulsion tubes that mix the air and fuel before it flows out of the discharge ports.
Engine tuners can find the optimum fuel mixture a variety of ways. The old fashioned way is to read the spark plugs. If the electrode insulators are black with carbon soot deposits, the air/fuel mixture is too rich. Install smaller main metering jets. If the electrode insulators are yellowish or blistered, the air/fuel mixture is too lean. Install larger main metering jets.
A far more accurate way to determine the air/fuel ratio is to run the engine on a dyno with wide band oxygen sensors installed in each exhaust header. A wide band O2 sensor will reveal the exact air/fuel ratio, which can then be adjusted by changing the jets in the carburetor until you find the sweet spot of about 12- to 13-to-1.
The dyno horsepower and torque readings will also tell you if you are getting more or less power with each change in carburetor jet sizes.
Sizing Up the Competition
Bigger is better, right? Size does matter with carburetors, but not in the way you may think. It’s all about the CFMs.
Carburetors are rated according to how many cubic feet per minute (CFM) they can flow. Four-barrel carburetors come in a variety of ratings, from 500 all the way up to 1100 CFM or higher. The most common sizes for small blocks (up to 350 cubic inches) is 600 CFM.
For stroked small blocks and smaller big blocks, a 725 CFM carb is often the best choice. For big displacement stroker motors (over 450 cubic inches), an 800 CFM or larger carb is usually needed.
How much airflow capacity is needed depends on the displacement of the engine, the maximum rpm it will turn, and what kind of cylinder heads and camshaft are used. A high revving (8000 rpm+) small block with large valve, large intake runner heads and a long duration, high lift cam can obviously handle a lot more carburetor than a low revving relatively stock engine.
You want a carburetor that has enough flow capacity to match the engine’s breathing potential, but you don’t really need much more. In fact, if you install a carburetor that has too much flow capacity on an engine, you can easily kill throttle response and low end performance.
You hear all kinds of discussions about engine breathing efficiency and how it affects airflow. Breathing efficiency changes with rpm and throttle opening, and may drop from 100% down to 80% or less on a stock engine at higher speeds. But on a performance engine with big heads, a hot cam, and free-flowing intake and exhaust manifolds, it may go as high as 120% or more depending on engine speed. That’s why dyno tuning is so important.
An engine makes the most power in the rpm range where it breathes most efficiently. So the key to maximizing the engine’s power potential is to make sure the carburetor is sized accordingly.
Retrofitting EFI To Carbs
Though some engine builders are installing aftermarket electronic fuel injection (EFI) systems on older engines, the opposite is also true. Why would you ever want to retrofit a carburetor onto a newer engine?
Aftermarket parts suppliers now have intake manifolds that allow late-model Chevy LS engines and Ford modular V8s that came factory equipped with EFI to be retrofitted with a carburetor. This makes it easier to install these late-model engines in older pre-computer cars, street rods and race cars where electronics may not be allowed.
We’ll finish up this piece with a few more cautions about carburetors. No engine will start, idle or perform well if the carburetor is dirty, damaged or screwed up. Carburetors on dirt track cars often develop wear around the throttle shafts that allow unwanted air into the engine. This can lean out the fuel mixture and cause hesitation and stumbling problems.
The rubber diaphragms, plunger pistons and gaskets in carburetors typically harden, shrink and/or crack with age. This can cause fuel leaks, vacuum leaks and flat spots if the accelerator pump and/or power valve are affected.
Dirt in a fuel line that gets into a carburetor can plug up metering jets, or prevent the inlet needle valve from closing, causing the carburetor to flood out.
The base plate on the carburetor may be warped or scratched, preventing it from sealing tightly against the intake manifold. This can allow vacuum leaks that affect idle quality, power and performance.
Used carburetors purchased at swap meets, on eBay or from persons unknown are a gamble. You don’t know what’s been done to the carb or the seller’s real reason for getting rid of it. A used carb may be totally screwed up because somebody has drilled out the jets or metering circuits, or modified other internal parts.
Bolting a bad carb onto a good engine can cause all kinds of headaches you don’t need. If you don’t know its condition, be sure to do a thorough investigation of it before adding it to your engine.
Of course, carburetors can be rebuilt and many businesses make a good living rebuilding, retrofitting and tuning performance carbs with aftermarket or OE parts. If you choose to do the work yourself, you can find suppliers of service and repair parts online in the Engine Builder Buyers Guide at www.enginebuildermag.com.
Another option is to find somebody who specializes in carburetor tuning and use them to set up the carburetor properly especially if your customer is reusing a carburetor or installing a carburetor off another engine. That’s often a prescription for trouble.
New carburetors are virgin in that they haven’t been tampered with, and many suppliers of aftermarket carbs can deliver a unit that’s very closely calibrated to the engine you are building. You (and your customer’s engine) will breathe easier if everyone has the right information.