Step 2 Unbolt the fuel bowl bolts from the metering block of the carburetor.If the engine shows performance problems such as stalling or sluggish acceleration, youll need to change the jet sizing.For example, a jet may be stamped (on its head) with the number 30.Installing a smaller size jet will reduce fuel flow, which is ideal for high-altitude conditions where air is limited, where a larger jet size increases fuel flow for oxygen-rich, low altitudes.
Holley Carburetors Jet Size Recommendation How To Correctly ChangeWith this knowledge, youll be able to understand how to correctly change the jets to regain the engine performance levels that were lost due to improper jetting. Jet Sizing For Altitude Changes Step 1 Remove the jets and insert a smallerlarger set based on the vehicles operating altitude. When the operating altitude of the engine will be 2000 feet higher than your normal altitude, youll need to insert a jet one or two sizes smaller than the size currently installed in the carb, which reduces the amount of fuel entering the engine to match the reduced level of oxygen in the air. If the altitude is dropped 2000 feet, youll need to insert a jet that is one to two sizes larger than the current size, increasing the fuel flow to match the higher oxygen levels. The overall goal is to maintain the correct airfuel mixture ratio for your specific engine. An incorrect ratio will show in decreased engine performance. Step 2 Unscrew the fuel bowl bolts with a wrench and pull the fuel bowl off the carburetor. Save the gasket that was pressed between fuel bowl and the carburetors metering block (what the fuel bowl is bolted to). With the bowl off, the jet(s) are exposed and can be identified by their brass color and center orifice. Holley Carburetors Jet Size Recommendation Driver Or PairStep 3 Unscrew the jet(s) with a screwdriver or pair of pliers and note the size stamped on the jets head. As mentioned in step 1, youll need to increase the jet size for lower altitudes, while a smaller jet size will be needed for higher altitudes. Make the size change in increments of one to two sizes at a time. Screw the new jet size into the jet mounts within the metering block, with a screwdriver or pair of pliers. Press the fuel bowl gasket and fuel bowl against the metering block and tighten the bowl bolts with a wrench. Turn the engine on and test it by listening for a smooth idle and a crisp throttle response. If the carb continues to under perform, remove the bowl and make another one to two jet sizing adjustment, either up or down based on your circumstances, until the engine runs well. Jet Sizing When the Temperature Changes Step 1 Install a smaller jet size if the ambient temperature dips below 35-degrees Fahrenheit. This creates a lean condition that runs the engine hotter, ideal for cold conditions. If the air temp raises above 105 degrees, larger jets will be needed to increase the amount of fuel entering the combustion chamber, known as a rich condition.
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