Diagnosing a carburetor issue

It is important to know that all a carburetor does is mix air and gas. The air is a constant according to the throttle position, unless you have a choke problem or a vacuum leak. Below is a process you can follow to test whether or not your vehicle has carburetor issues.

When you have a carburetor problem it will be because you either have too much gas (rich), or not enough gas (lean) being mixed in with the air. If you see black smoke coming out of your exhaust, you either have a stuck choke, bad choke pull-off, or are not getting enough heat to your choke in order for it to move to the off (open, vertical) position. If your choke is operating properly and you see black smoke, your carburetor is very rich and needs to be rebuilt.

There are 7 circuits in most carburetors, but we will concern ourselves with 4 areas for diagnostic purposes: the idle circuit, off idle circuit, main circuit, and accelerator pump. When you check your carburetor, remove your air cleaner and plug any vacuum lines that were connected to it. When working on a running engine you will want to take the proper precautions of staying well clear of all moving parts, wear safety goggles, and make sure the car is in park or neutral with the parking brake set, or wheels blocked. Never look down the throat of a carburetor with the engine running. The following tests should only be done by someone who has some experience working on cars.

The first thing you want to do, is see if the problem can be corrected with an adjustment (read the article: “Adjusting your carburetor at idle”). If not, is the carburetor too rich, or too lean? To determine this, introduce an additional fuel source (like carburetor cleaner spray) by spraying into the throat of the carburetor with the engine idling. If the idle smoothes out and gains RPM, you have a lean condition. If no improvement, or it gets worse, it is rich. If it is too rich the best idle will be achieved with the mixture screw turned all the way in (never torque a mixture screw tight, only lightly screw it in to find the seat). If your idle is still rough with the mixture screw turned in and it’s not lean you will likely need to rebuild your carburetor. If it is too lean the best idle will be achieved with the mixture screw(s) turned out between 3 to 5 turns. If you have the mixture screw(s) turned out this far and you still have roughness or stalling, and it responds to carb spray, then you have leanness that is beyond adjustment and you either have a vacuum leak (see paragraph at the end for how to check for a vacuum leak), or the carburetor needs to be rebuilt.

With the engine off look down the throat of the carburetor and move the throttle at a medium speed. On a one barrel carburetor you should see a stream of gas squirting in with the movement of the throttle. On a two or four barrel carburetor you should see 2 streams of gas squirting in with the movement of the throttle. If there is no squirt or if the squirt is weak, then the carburetor needs to be rebuilt. If the squirt looks good, the next thing you are going to do is check for off idle or main circuit leanness. Start the engine and after it is warmed up hold the throttle steady between 1,400 and 1,800 RPM. Spray carburetor cleaner into the throat of the carburetor. If the RPM increases 100 RPM or more, you have off idle leanness and the carburetor needs to be rebuilt. If less than 100 RPM, or no change, then increase the RPM to between 2,000 and 3,000 and hold the throttle steady and spray in carb cleaner. An increase of more than 100 RPM and the carburetor needs to be rebuilt or re-jetted.

A carburetor that is putting in the proper mixture will tolerate the addition of a small amount of extra fuel. If there is richness, it will lose RPM when more fuel is added. You can also look at a spark plug for an indication. If all the plugs are black, you have an overly rich condition.

With the engine warmed up and idling, spray your carb cleaner into the throat to get a smooth idle. Then, back off on your idle set screw to lower your idle while continuing to spray into the carburetor. Keep backing off the screw until it is no longer contacting the throttle linkage. By doing this, you are taking away air and your car should stall, or at least idle down very low to 600 RPM or lower. If this is the result you get, then you are not getting enough fuel thru the idle circuit and the carburetor needs to be rebuilt. If the RPM stays higher, check to make sure that nothing is hanging up the throttle.

If you are sure the throttle is closing and the idle is still high, that’s an indication that the engine is getting air from some other source and you have a vacuum leak. To check for a vacuum leak, take a pair of needle nose pliers and squeeze each vacuum hose that is connected to the carburetor and intake manifold one at a time. If you get an RPM change or the idle smoothes out, then you have a vacuum leak in that hose or the component connected to that hose (note: when the PCV valve hose is squeezed you will get about a 50 RPM drop – this is normal).

If you don’t find any leaks in the vacuum hoses, you can spray the carb cleaner around the base of the carburetor and where the intake manifold connects to the cylinder head watching for an RPM change. This test however, can give you a false reading because the spray will vaporize quickly when it hits a hot surface and get pulled into the carburetor. So it is best to do this test before the engine is warmed up. The other possible causes of the idle remaining high would be a worn out throttle butterfly, or a throttle shaft that needs a bushing, or, if it’s a four barrel, sticking secondary butterflies.