Holley gasoline carburetors are designed to bring fuel into the engine in metered increments. If you upgrade your engine to run alcohol, your gasoline carburetor will need to deliver twice as much alcohol as gasoline. To make this upgrade, you'll increase the jet and fuel nozzles, drill out ports on the metering block and increase the flow within the fuel bowls. You'll need to upgrade both the primary and secondary components of the 4-barrel 4150.
Unscrew the fuel bowl from the metering block.
Pull out the fuel bowl needle and seats using a pair of pliers.
Clip the larger needle and seats into place. During this process, you may opt to replace the plastic float with a brass alternative by un-clipping the float from the float valve hinge and clipping on the brass replacement.
Repeat this process on the identical secondary float on the rear portion of the carburetor.
Remove the two brass well plugs with pliers. These "wells" are the largest identical features on the metering block. If the brass plugs are not present on your specific 4150, use a 1/4-inch drill bit and a drill press to drill down 1/4 inch into the well plug shaft. You must use a drill press for this step to ensure the hole is straight. Note that some 4150s come with plastic "emulsion" tubes within the well shaft. If you see these, remove them. Its okay to destroy these plastic tubes upon removal, as you will not need them.
Locate the angle ports that feed from the main wells to the booster supply passage, identified by their inward-facing angle. Drill them out slightly, taking care not to drill through the passage and into the well shaft, using a #15 drill bit.
Replace the standard size jets with the #100s in the metering block. Once installed, drill a hole above each jet, using a #42 drill bit until it breaches the other side of the metering block. This will increase the fuel flow above what the #100 jets will offer, allowing you to adjust the jet size down later, if a leaner fuel mixture is required.
Locate the idle circuit on the side of the metering block and ream the passage larger using a #53 drill bit. This will provide more fuel during idle conditions.
Remove the power valve and its O-ring. This component is not necessary on race vehicles running alcohol.
Clean the metering block with carburetor cleaner spray and repeat this process on the secondary metering block.
Use a #6 drill bit to increase the size of the booster walls. The boosters (one on each side of the metering block) are just above the power valve slot.
Pull out the two fuel nozzles located in the throttle bore area, using a pair of pliers.
Replace the nozzles with a set of #45s and swap the nozzle screw with a set of hollow screws to increase the flow of fuel even more.
Locate the idle bleeds (the two outside holes within the secondary throttle bore) and ream them out using a #33 drill bit. Note that the inside holes are the high-speed bleeds and do not need to be drilled.
Clean all components of the carburetor, reassemble the metering blocks to the choke tower and reattach the primary and secondary fuel bowls. This completes the upgrade and the carburetor is ready to be mounted on the engine and tuned to your engine.
