5 Considerations When Choosing a Carburetor

Are you on a low carb diet?

I am not going to go into too much detail regarding carburetors.  In fact, currently the trend is turning more towards fuel injection which is arguably a more reliable system, but requires a lot more electronics and computer control.  However, one thing that is certain is that no two mechanics have ever had the same opinion about what fuel delivery system is best.  One mechanic will tell you thatRochestercarbs are the only way to go, the next guy will say that Holley is the best.  In all the experience I have till this point, I have never had anything but bad luck with carburetors.  And in the end, I always revert back to the advice of a good friend of mine, a superb mechanic, and perhaps the smartest man to whom I am acquainted. He told me that whatever carburetor you choose, either buy a new one, or have the old one professionally rebuilt.  This is one of the few instances in this book that I will recommend spending money on something.  But carburetors are one of them.


To be fair, it requires a little explanation.  Carburetors are actually a wonder of mechanical genius.  It would seem so simple to build a device that simply sprays a stream of fuel into a stream of air, but there is so much else going on inside a carburetor that it defies the imagination. Levers and pulleys and floats, vacuum and pcv inlets, chokes and throttle plates; all working together in unison to balance the fuel and air intake in such a way as to provide your throbbing engine with such sweet nourishment.  But if any one of the hundreds of tiny parts that make up your carburetor is even slightly off, the effects can be constantly aggravating to say the least and catastrophic to say the worst.  The simple truth is that a rebuilt carburetor will have been put together and TESTED at a facility, or by a person possessing all the specialized tools and equipment necessary to assemble and set up a carburetor properly.  When a carb is rebuilt, the first and often times most important step is to take the unit apart and chemically strip the varnish or gasoline residue from all the inside parts.  And despite what the local parts clerk tells you, the carb cleaner in the spray can does not do a good enough job.  Carbs need to be acid washed in special, sickeningly toxic chemicals in order to properly remove the gasoline residue from all the tiny orifices.  And if this first step is not accomplished completely and properly, the effects are often an aggravating stall or flat spot in your rpm when the carb is reassembled and back on the motor.  What makes carburetors particularly difficult to work on is the fact that they are under constant vacuum pressure.  In effect, if a carburetor is not sealed properly, it will suck in outside air and oftentimes result in a high idle.  (Oh by the way, if you are ever trying to figure out why your engine will not idle down, look for a vacuum leak)


Here is some advice about choosing a carburetor:  Think of the best mechanic you know and one with whom you are well acquainted.  Hopefully this is a guy that will come over to your garage and help you set up your carburetor when the time comes; if possible at no charge to you.  Ask him what fuel system he would recommend.  Then if he does not elaborate, ask him why he prefers that particular system.  The most difficult part about carburetors is setting them up and tuning them.  And, while there are undoubtedly whole libraries full of books that explain carburetor details down to the finest adjustments, nothing compares to calling on the experience of a veteran mechanic who is intimately familiar with a particular carburetor.  I would point out that all the technical carburetor manuals in the world will not account for some of the obscure malfunctions that can occur when trying to tune your engine up.  And a seasoned mechanic can oftentimes tell you the problem just by listening to you rev up your motor or by taking your rod for a quick spin around the neighborhood.


I will submit at this point that I have learned these lessons from years of excruciating misdiagnoses while trying to get various vehicles to run properly.  Most mechanics know how carburetors work in general.  But unless they have had specific experience with one particular brand or model, they can only apply their general knowledge to the proper tuning of a carburetor.


Besides carburetors, there are many other fuel delivery systems.  Most of them are significantly more expensive and complicated to install.  The most common being electronic fuel injection, EFI for short.  Fuel injection is not a new concept.  Most all diesel engines are fuel injected as demanded by the physics of diesel combustion.  So the technology is very reliable and has been for a very long time.  In the category of fuel injection in a gas engine, there are two main varieties.  The first and most comparable to carburetion is throttle body injection or TBI.  The term is fairly self explanatory.  Where the carburetor would go on a normal engine, a throttle body injection unit would go on a TBI engine.  The second and more expensive variety of EFI is TPI which stands for tuned port injection.  This system is a little more complicated than the previous one because in TPI the fuel is injected into the ports or for lack of a less complicated explanation into each cylinder.  So, as you will soon guess, the injection of fuel into each cylinder has to be timed appropriately to coincide with the firing of that cylinder.  All things considered

This method has the benefit of delivering fuel almost directly to the cylinder, but has the disadvantage of requiring a complex control system to ensure that the operation of the injectors is appropriately timed.  Also, some of you may ask what the benefit is to having the fuel delivered directly to the cylinder instead of into the airstream at the intake manifold opening.  The answer is balance.  Remember our discussion earlier about obtaining the maximum power from an engine when all the cylinders are equal and operating in unison at high rpms.  Well the theory with tuned port injection is that the amount of fuel reaching each cylinder can be made almost equal.  Not that carbureted or TBI systems don’t deliver approximately the same amount of fuel to each cylinder, but there is a tiny difference.  When a TBI system or a carburetor atomizes fuel at the opening of the intake manifold, somewhere between that point and where the fuel/air mixture gets to each cylinder, there may be some losses.  Fuel may condense on the walls of the intake manifold, or one of the passages from the throttle body to the head may have a slightly larger volume and shape than the rest causing the fuel air to flow more slowly and eventually deliver less fuel to that cylinder.  Sounds pretty far fetched doesn’t it?   Well in most cases it is.  Most hot rodders would never know the difference.  So at the end of the day, it really depends more on what your budget allows and what your skill set demands.  Tuned port injection units look cool, but a nice shiny chrome carb looks pretty good too.  So let’s move on and examine some other differences between carburetion and fuel injection.


Another significant difference between the two systems is that a carburetor uses the vacuum pressure of the engine, the fuel pressure from the inlet and a complicated balance of fluid pressure interchanges and mechanical devices to deliver a stream of atomized fuel into the airflow entering a motor.  An EFI uses a computer to decipher the information from several sensors (MAP, MAF, O2, RPM, and throttle position, amongst others) and appropriately control an electronic injector that literally sprays gasoline into the intake air stream.  So what is the difference between the two?  Well, a carburetor can be thought of as self sufficient.  Once it is set properly, it will deliver the appropriate amount of fuel according to the engine load, rpm, and throttle position.  Simple as that.  It is almost like a computer itself.  Once you have it programmed to function in the manner that you like, it will continue to operate in that fashion as long as the fuel is supplied and the ignition is on.  A fuel injection system relies on information that is obtained from several different sources in order to decide how much fuel to deliver.  Because the injection is electronic, there is no direct link between the gas pedal and the fuel delivery.  In essence, there has to be a computer or some sort of electronic controller in between to modulate the flow of fuel through the injector nozzle.


Let’s backtrack a bit and look at where our fuel is coming from.  First, a carburetor is usually fed gasoline by a mechanical fuel pump powered by an auxiliary lobe on the camshaft.  Some carbureted vehicles have electric fuel pumps, but most of them have mechanical pumps.  For the fuel supply requirements of a carburetor, a mechanical fuel pump does the job quite nicely.  However, since the pump is being run off a mechanical component in the engine, it tends to speed up as the motor revs higher.  As well, most mechanical fuel pumps are of a ‘stroke’ variety.  That is, they operate like the water pump on a well.  One stroke does the actual pumping and the backstroke gathers fluid for the next pumping stoke.  The end result is that mechanical fuel pumps tend to provide fuel with an inconsistent and often throbbing pressure.  And, for the most part, carburetors don’t mind.  A carburetor body is usually holding a significant amount of fuel at any time.  The carburetor accepts the fuel as is needed and is unaffected by slight variations in the fuel pressure.  Fuel injection systems are not so tolerant however.  The fundamental concept of fuel injection is that an injector can be electrically controlled to release a very precisely measured and beautifully atomized spray of gasoline.  These injectors are not so different from the squirt bottles that Windex comes in.  Now think about that Windex bottle.  If you pull the trigger slowly what happens?  Usually the Windex doesn’t come spraying out like it’s supposed to, it just dribbles out like an old man in front of you at the ball game urinal.  The same theory applies to fuel injection.  In order for the electric injector to spray consistently, the fuel pressure has to be very uniform.  As well, fuel injection pressures are usually significantly higher than carbureted systems.  So, in order to accommodate the consistent and high fuel pressure, fuel injection systems always require an electric fuel pump.  In most cases, the fuel pump is located very close to or inside the fuel tank.  There are several reasons for mounting the pump inside the tank.  The first is that the tank does a wonderful job of muffling the noise that an electric fuel pump will make.  Second, with the pump mounted directly in the tank, there is nothing to impede the flow of fuel before entering the pump.  This may not sound like a significant factor, but in terms of fluid dynamics that are beyond the scope of this blog, it is very beneficial to have the pump submerged in the fuel tank.  


Now let me back up a bit on fuel injection.  I said before that the fuel delivery is based on input from a number of different sensors.  This may not be quite the most accurate way to describe fuel injection as it pertains to the possible installation on a hot rod.  However, this is the way that fuel injection systems work on newer, production vehicles.  The main reason why fuel injection took over from carburetion was due to a much higher capacity to control emissions.  The computers on modern vehicles receive information from numerous sensors located not only on the engine, but everywhere on the vehicle.  The O2 sensors are usually located on the exhaust; oftentimes there are two, one before the catalytic converter and one after it.  These two sensors in combination provide information that the computer uses to determine whether the more harmful emissions are being processed properly by the catalytic converter.  The computer would be programmed to alter the fuel/air mixture to maximize the effectiveness of the catalytic converter and therefore minimize noxious chemicals in the exhaust.  As well, many of the other sensors provide information that the computer uses for the same purpose.  A MAF (mass air flow) sensor tells the computer how much air is flowing into the motor and most often also senses the temperature of the air.  The computer would use this information to alter fuel flow during cold winter months, or blazing desert highway miles.  Even altitude is sometimes taken into consideration.


So what does all of this mean to you the hot rodder?  I don’t want to discourage anyone who wants to install fuel injection on their hot rod, but be prepared to get high tech.  Some new manufacturers may package fuel injection units that are fairly easy to install and tune, but I have never personally had much experience with them.  In terms of installing a fuel injection system that has been scavenged from some other vehicle, be prepared to remove all the sensors and computer systems necessary to ensure that the injection system functions properly.


All that being said, I will now take a moment to summarize the important facts to keep in mind when selecting a fuel delivery system:


  1. Carburetors are a complicated and finicky device.  If you want to take a crack at rebuilding one, go ahead.  But don’t be surprised if it never works properly again.  It is better to spend some of your carefully allotted hot rod budget to purchase a brand new or professionally rebuilt unit.
  2. Tuning a carburetor is sometimes a fine art.  There are many books available to guide you along, but nothing will replace the expertise of a veteran mechanic.  Try to find an experienced mechanic that can help you select, install and tune a carburetor without charging you $80 an hour.
  3. Fuel injection comes in several configurations and price ranges.  Fuel injection has the advantages of being very reliable and highly tunable, but capitalizing on these qualities often requires the user to become familiar with some complicated electrical and computerized components.  There are, however, companies that are manufacturing fuel injection kits that are supposedly bolt on and quite user friendly.
  4. Due to the computerized or electronic nature of fuel injection and the substantial number of sensors often required to make these systems work, it is often difficult to remove and utilize a fuel injection system from a newer vehicle.
  5. Remember, fuel injection might also mean an in-tank fuel pump, which might mean a custom tank.  All these things are going to cost money.  Make sure your budget allows for all these support components before choosing a fuel injection system over a plain old carburetor.

Hopefully there is some information in this article that might help you decide which way to go in terms of fuel delivery, but look for more in depth articles that explain each option in more detail.  And don’t forget to tell your friends about howtobuildhotrods.com