What are carburetor jets
In simple terms, jets are small threaded plugs with a tiny metering orifice throughout its center that control the amount of fuel entering the carburetor’s venturi. They are considered metering components usually made from brass. The diameter of the main jet metering orifice determines the fuel flow rate, hence the slightest difference could alter the air-fuel ratio. Although a carburetors’ jet comes out-of-the-box pre-calibrated for normal conditions, it can be replaced if necessary with another jet with a different hole diameter.
How do they work?
As air goes down through carburetor venturi it creates a low-pressure zone at its thinnest point where the main jet is located. Such vacuum pulls atomized fuel from the fuel bowl through jet’s metering orifice allowing it to mix with air at the venturi creating one homogeneous air-fuel mixture. Since jets are responsible for allowing fuel passing from the bowl to throttle bores, their diameter has a direct impact on fuel flow. A larger diameter will enrich air-fuel mixture while a jet with a smaller diameter will have the opposite effect, creating a lean air-fuel mixture.
When do you change jets?
As mentioned earlier, carburetors are calibrated at the factory for ‘optimal performance under normal conditions’, now, what are normal conditions? For most manufacturers ‘normal conditions’ are 70°F at sea level. What that means is, that depending on the area where you live, you might need to change jet size. Moreover, there are additional factors that might force you into changing jet sizes. Some of these factors are listed below:
Weather (higher or lower ambient temperature, humidity levels)
Barometric pressure (higher or lower altitudes affecting oxygen levels). 5,000 feet above sea level will usually require jets be downsized by .002.
Upgrades to the engine (cam profile, cylinder heads, intake manifold, turbo, supercharger)
Changes to the exhaust system (headers, low restriction exhaust system, open exhaust)
Normal wear over time - This is seldom an issue because jets don't wear from normal use.
Now, the logical question is, how can you possibly determine if changing the main jet is really necessary? How to know if It would be necessary a smaller or larger diameter?
Fortunately, there is a pro tip that can assist you in answering those questions which consist of ‘reading’ the spark plugs. When the carburetor is properly ‘tuned-up’ spark plugs should be completely dry and have a light-brown/tan color. When spark plugs show that condition what they are telling you is that the air-fuel mixture is adequate. On the other hand, a spark plug with bright-white ceramic and/or electrode is indicative of a lean condition meaning that you need to increase the jet size. Usually, increasing .002 inches is a good starting point, however, you will need to test the vehicle and check again to verify the air-fuel mixture is correct.
NOTE: Poor performance from the engine electrical system can also affect the spark plug burn.
Increasing the jet size allows more fuel to reach the venturi thus you are compensating the lean condition. Notice though that a vacuum leak, a faulty fuel pump, or a clogged fuel filter may also cause a lean condition, so be sure to check those issues first.
NOTE: A lean condition can be especially bad for engine wear.
What about a rich condition? When the spark plug is black, like covered by soot, and it’s not completely dry (it usually smells of gasoline) then you have a rich condition. Fixing a rich mixture can be fixed by decreasing the jet size by .002. Same as before, you will need to perform a complete test drive between each check. Please notice that ignition problems could also cause this condition. Check spark plug wires, distributor cap, rotor, and ignition coil as needed.
When performing test drives between checks try to do it at cruising speed of 35 mph or more for several miles. This ensures proper spark plug readings. Also, if you are driving a V-shaped engine then remove one spark plug from each side of the engine to obtain better results.
Where is the jet size found?
Usually, jet size is engraved on its surface. Depending on the manufacturer, the number may indicate its diameter in millimeters or thousandths of an inch, but also may be an arbitrary number meant to be used with manufacturers charts to find its flow rate.
An example is Motorcraft. You might find 50F stamped on the jet. This relates to .050.
How do you measure a jet?
Since jets’ defining aspect is its diameter, you will find special jet size gauge tools on the market. Depending on the carburetor manufacturer, the gauge tool may measure millimeters or thousandths of an inch. Using the jet sizing tool is rather easy. You just need to insert the tapered needle into the jet and read its size from the scale, or in the fixed type gauge, find a needle that fits in the jet and then read its number.
The illustration above is using an outside caliper to measure the jet hole. This isn't the most accurate way, but will get you very close. There is also many jet sizing tools in the market that are in metric. This will also get you real close. You just need to convert to decimal.
What if I can't find jets for my carburetor?
Some jets are no longer produced. In this case you can do 1 of 2 things.
1 - To make the jet size smaller, solder the hole shut and re-drill the size you need.
2 - To make the jet size bigger, drill the hole bigger.