Fuel injection’s been standard equipment on new cars for something like 20 years now – so few under-30s have much hands-on experience with carburetors. But since virtually all classic-era cars had carbs and not FI, learning about carbs is as essential as understanding VIN codes – if you plan to keep the car original, anyhow.
Here are ten basic things to keep in mind about carbs — how they work, what’s involved — and what things you should never do to one.
1) Carburetors Are Delicate
Most were made of an alloy such as aluminum – and unlike the cast iron intake manifolds and engines onto which most of them were bolted, they are easily damaged by overtightening their mounting bolts or being manhandled in other ways. When installing a carburetor onto a cast iron manifold (or an aluminum manifold, for that matter) be gentle – and use a torque wrench – or you risk warping the carb’s base plate, which can lead to annoying vacuum leaks, poor sealing and lots of headaches – including a ruined carb!
2) Choose The Right Carburetor
Some carburetors are better-designed than others; or at least, better designed for a given purpose. For example, Holley carburetors are superb for all out performance – but not the best as far as everyday driveability goes. The Rochester Quadrajet, on the other hand, is an excellent all-arounder, although it is more complex and thus harder to tune for the novice than a Holley. Most any carb can be made to function on most any car – but that doesn’t mean it’s the best choice for a given car. If you’re new to the carburetor scene, ask someone older/more experienced for advice. And if you are buying an aftermarket carb, be sure to ask the technical advisors from the company that sells the carb lots of questions so that you’ll get one that’s closest to right rather than one-size-fits all.
3) Adjust Just One Thing At A Time
Most carburetors feature multiple points of adjustment to “dial them in” for a given application. These adjustments include such things as idle speed and mixture, float level, primary and secondary jetting, accelerator pump shot/duration, secondary air valve opening rate – and so on. Before you make any changes at all, everything should be “baselined” to factory stock specifications for the year/make/model of car you’re fiddling with – assuming the car itself is stock. If not, you’d still be well-advised to make only one change at a time and note the effect on operating characteristics. If it makes the car run worse, go back and try again. And always make notes of what you do – so you can easily undo it if you have to!
4) Watch Out For Caustic Modern Fuels
Most carbureted vehicles were designed years before the introduction of alcohol-based additives such as MTBE (and E85) and may have internal parts (such as floats) that won’t stand up to modern fuels. It’s a good idea to replace old-style foam floats, for example, with brass ones. These have the added benefit of not becoming saturated and leading to incorrect float bowl levels (either too much fuel – or not enough). You definitely want to avoid old-style floats deteriorating and shedding bits of debris that could clog small passageways and orifices (such as jets) in the carb. It’s a good idea to rebuild any old carb with modern gaskets and other parts designed to handle today’s fuels. Most rebuild kits will have everything you need to do the job – and update your carb.
5) Be Careful With Electric Fuel Pumps
Unlike modern EFI, which operates at very high pressure (32 psi and up, in most cases) most carbureted systems are designed to operate at comparatively low fuel pressure – 3-5 psi or so. Anything much higher than that can overcome the needle and seat assembly (which regulates fuel flow into the carb) leading to flooding and terrible driveability. The problem sometimes arises when an older, carbureted car is fitted with an electric fuel pump in place of the original mechanical one. Electric pumps usually deliver much higher line pressures than mechanical pumps, so if you’re using one, be sure to also use a regulator to keep the pressure at the carb within the factory recommended specs.
6) It Is Better To Run Too Rich Than Too Lean
An over-rich mixture (too much gas) may lead to plug fouling, terrible mileage, high emissions and rough running – but usually won’t cause any major, permanent mechanical problems. However, an over-lean mixture (not enough fuel) can lead to serious problems, even major engine damage – and so ought to be avoided. Err on the side of too much gas rather than too little. The planet may not be happy – but your older car will be.
7) Avoid Any Modification That Can’t Be Undone
Examples here include anything that involves a drill bit or bending/breaking off a piece of the carb – which some “experts” might recommend to improve performance. Maybe it will. But if it doesn’t (or you do it incorrectly) the damage is forever and you’ve just ruined the carb. Tinkering with different jet sizes, metering rods and so on is a big part of the fun of carburetor tuning. But it’s nice to know if what you’ve done doesn’t work well that you can always go back to where you started – no harm, no foul. And these days, carburetors are no longer a dime a dozen, either. Ruining a rare, numbers-matching casting can be a real downer – as well as a big hit to your wallet.
8) Do Not Forget The Return Spring
When you stomp on the gas pedal, a cable pulls back the throttle arm on the carburetor, opening up the primaries – and the secondaries – for all-out performance. But sometimes, the linkages can get sticky – which would mean a throttle that stays wide open if you’ve taken your foot off the loud pedal. Unless the return spring’s in place. This little spring is a cheap fail-safe designed to exert opposing pressure on the throttle arm/linkage, so that when you’re no longer pressing down on the gas, the throttle snaps closed, as it ought to – and you don’t drive through a plate glass window and into the shopping mall. Never remove the return spring – or “modify” it in any way. It’s there for a reason – a very good reason!
9) Keep The Carb Clean
Modern FI systems are to a great extent “closed” systems in that the injectors, fuel rail and other components are not directly exposed to engine compartment grime or airborne dust from outside. But carbs are pretty exposed; the “throat” is only protected by the air cleaner – and it’s much easier for small particles, dirt, debris and so on to get into places it shouldn’t. Part of keeping a carb happy involved keeping it clean, externally and internally. The exterior should be sprayed own with an aerosol carb cleaner as often as necessary to keep the surfaces clean and free of grime; the same cleaner should be sprayed into the carb’s throats to keep the main wells/jets and so on clean, too. Also be sure to regularly change out the fuel filter and (of course) the air cleaner element. Never run the engine without the air cleaner installed, except for making adjustments in the garage.
10) Keep A Fire Extinguisher Handy
One thing about carbs that can be dangerous is the presence of raw gas in and around ignition sources – including possible flash fires caused by fuel spilling onto hot engine parts such as exhaust manifolds. A backfire through the carb (without the air cleaner installed) can also lead to an automotive immolation. It’s a good idea to keep a fire extinguisher around for any car – carbureted or fuel injected. But with carbureted cars, the risk’s a little higher and it’s an especially smart move to keep an extinguisher in/with the car – vs. just hanging one on the wall of your garage. Many old car shows actually require each vehicle on display to have an extinguisher available – just in case.
This is a guest post by automotive columnist Eric Peters, check him out on the web at www.ericpetersautos.com.