by Jim Kerr

Carburetted engines are a thing of the past. Fuel injection started to appear on many vehicles in the early 80’s, and by 1990 almost every vehicle sold was equipped with fuel injection. In some parts of the country, there are many carburetted vehicles still on the roads and most will continue to be excellent vehicles for many years. As these vehicles age however, small problems begin to show up that can make driving a chore. One problem area is the carburettor.

Fuel injection has made starting an engine easier. Usually you just turn the key and the engine jumps to life after only one or two revolutions. Carburetors, on the other hand, require the engine to be cranked for several revolutions before the fuel reaches the cylinders and the engine fires up. If everything is not perfect on a carburetted vehicle, then the engine may not start due to too much or too little fuel.

When an engine won’t start because it is getting too much fuel, it is referred to as a “flooded” engine. Holding the accelerator pedal to the floor and cranking the starter will eventually get the engine running, but it will probably sputter and runs rough until heat is built up in the engine and the engine is no longer flooded.

Many people attempt repairs by bending or adjusting the carburetor linkage. Seldom do carburetors go out of adjustment. If they were working correctly once, then the adjustments are usually satisfactory.

Three problems commonly occur with carburetors. The first two are the linkage becomes sticky, or a passage inside the carburetor becomes plugged. Disassembling the carburetor, cleaning it, and installing a new carburetor kit is necessary if this occurs. The third problem is a part wears or breaks. Replacing the part is required, but the carburetor does
not usually have to be disassembled.

When a carburetor causes an engine to flood during start up, the fuel level is too high, or a vacuum break is not working correctly. Too high a fuel level inside the carburetor will cause the engine to run rich all the time. This can be noticed by a decrease in fuel economy, and perhaps black smoke coming from the tail pipe all the time the vehicle is driven.

The fuel level is controlled by a float and valve (needle/seat) inside the carburetor. If you want to understand how it works, just remove the cover from your toilet tank and flush several times as you watch the water enter the tank. Carburetors use the same type of system on a smaller scale. Floats can soak up fuel over a long period of time and become too heavy. When this happens, the fuel level goes too high. Many technicians automatically replace a float during a carburettor rebuild, but if you want to check it, they usually weigh between 7 and 9 grams for a single float system. This is approximately the weight of four dimes.

A faulty carburetor vacuum break is often the cause of a flooding problem. The vacuum break is mounted on the outside of the carburetor and its purpose is to open the choke slightly when the engine is first started. It has a small rubber diaphragm inside a housing and when engine vacuum is applied to one side of the diaphragm, it will pull the choke partially open. Some carburetors have two vacuum breaks: a primary one and a secondary one.

The primary vacuum break opens the choke as soon as the engine starts. The secondary vacuum break has a small controlled vacuum leak in it so it delays opening until 10 to 30 seconds after the vehicle is running. Then it opens the choke a little further than the primary break did.

The vacuum breaks can be checked visually by watching for movement when the vehicle is started. The air cleaner may have to be removed to do this so be careful of engine backfires. Do not look into the carburetor while cranking the engine and be sure to have a fire extinguisher handy. Remember that the primary vacuum break will move immediately, but the secondary one will take a few seconds to move.

Replacing the vacuum breaks are easily done just using a screwdriver. When purchasing a new one, take the carburetor part number with you to the parts store because there are hundreds of different vacuum breaks available. They may look the same but don’t work right due to internal spring calibrations. An adjustment will be required to fine tune the choke opening after the new vacuum break is installed, but it will usually be close right out of the box.

Connect with