Your carburetor is the heart of your car’s fuel system and is responsible for delivering the correct mix of fuel and air into the combustion chambers to keep your engine running at its peak. Although it’s built to last, tweaking your carb from time to time will help you avoid problems like erratic acceleration, rough idling and reduced fuel economy and power output. Tuning your carb is also necessary before firing up an engine with a new carb or one that’s been rebuilt.
Depending on the complexity of your carburetor, you may have different tuning options available to you. For instance, entry-level carbs often have the least tuning options like the idle speed, mixture and primary main jets while higher performance carburetors introduce a nearly endless number of tuning points. But regardless of your carburetor, getting just the base adjustments done correctly will give you noticeable results and if you decide to tackle more advance options, you’ll be able to do so with far less headaches.
All you really need to properly adjust a carburetor is a small plain screwdriver. However, if you have a vacuum gauge and something with a tachometer in it, that’ll help quite a bit. A tailpipe sniffer is also useful to get the adjustment exactly perfect, but it isn’t absolutely necessary to get within the ballpark. By smell, sound, and feel you can get the adjustment close enough to pass smog and get your engine running efficiently.
1. Set the Air-Fuel Mixture
Down at the bottom of the carburetor, usually on the front, you’ll find a small screw. If you have a four barrel carb, you’ll find two. Technically, this is called the idle-air-fuel mixture ratio adjustment valve/screw. We just call it the idle-air adjustment. It determines the quantity of mixture entering the engine through the idle circuit. Carefully turn it clockwise until it just stops. BE GENTLE! Don’t tighten it or you can ruin it. Now, turn it back out 1/4 turn. This will be our starting point. Of course, if you have a four barrel, change “it” to “them”.
2. Set the Idle Speed
Start the engine and let it warm up completely. Take a look at the tach. If you have an automatic transmission, you’re going to want your idle somewhere very close to 800 RPM. If you have a manual, you’ll want it closer to 750 RPM. Look around the engine compartment for a smog decal, it’ll give you the exact engine speed you’re shooting for.
Locate the throttle linkage on the driver side of the carburetor. You’ll see a screw there. Turning that screw clockwise makes the engine speed up and turning it counterclockwise causes it to slow down. Turn it slowly. This is an adjustment where a little goes a long way. Ideally, if you’re using the tach in the dash, you’ll have someone watching it for you, so you don’t have to run back and forth.
3. Fine Tune Your Initial Settings
Next, go around to the back and take a look and a sniff of what’s coming out the tailpipe. If your idle-air adjustment is off, you’ll be able to tell. You’ll smell the unburned fuel if it’s too rich. The exhaust will smell like something is burnt if you’re too lean. You can also monitor the temperature gauge. If the temp climbs higher than you’re used to, you’re too lean and need to open the idle-air a little more, like 1/8 of a turn. If it smells like unburned gas, you’re too rich and need to close it 1/8 of a turn. If you have to adjust the idle-air mix, double check the tach and readjust the idle speed as needed. The goal is to set the idle to burn in the leanest condition possible without causing the engine to sputter which causes the highest amount of vacuum pressure to be built up before the accelerator pump kicks in.
4. Test Drive and Adjust for Power
Now we have the fun part: The test drive. Get in and drive it. Jump on it some. Does it want to get up and go? Good, everything’s adjusted properly. No? Go back and open the idle-air 1/8 of a turn and check again. Never go further than 1/2-5/8 of a turn out, though. If you need to go farther, something else is wrong.
5. Check for Vacuum Leaks
If you’re still not satisfied with how it’s running even after adjusting everything properly, a vacuum leak could be the culprit. With the engine running, spray carb cleaner around the base of the carb and the intake manifold where it meets the head(s). If the engine stumbles or speeds up, you’ve found a vacuum leak. Next, lightly grip the throttle linkage near the throttle rod. Move it back and forth and then up and down. If there’s any movement, you need to pull the carburetor and have the throttle shaft shimmed, because it’s causing a vacuum leak. Next, closely examine all of the vacuum hoses for cracks. If you find any, replace the hose.
Depending on the complexity of your carburetor, there may be other adjustments you can make to get the most out of it but even now you should have an efficiently running engine that responds quickly to the accelerator pedal and doesn’t lag under pressure.
Photo credit: kevincole