Idle feed restrictors circled limit the total amount of fuel available to the idle and transfer slot circuits. Cover one of the idle bleeds and the rpm will pick up a little.
Carburetor Tuning The Scientific Way
Holley idle feed restrictor tuning. Make one adjustment only to the first screw. Make sure that your idle speed adjustment screw is set low enough that the primary throttle plate doesnt uncover more than060 inch of the transfer slot as viewed from the bottom side of the throttle body. Make very slight adjustments here as the factory setting is very close to ideal. Idle feed restriction idle jets can be located in different positions for different metering blocks. To access the carb must be inverted. The adjustment should be no more than an eighth or quarter turn.
He was able to tune his transition circuit with t slot restrictors adjustable idle feed restriction and idle air bleeds to cruise at 151. By the end of the thread he was able to acheive 17mpg on the highway. While this sounded bad. This is the idle stop adjustment for most holley carburetors. Set the idle mixture to the highest vacuum by using a vacuum gauge connected to the constant vacuum port of the carburetor base plate. On the right yellow arrow is the replaceable style for the ultra range of holley carbs.
Slowly adjust the first idle mixture screw. On the left red arrow is the fixed type most commonly seen in regular metering blocks. The standard idle feed restrictor size for most 750 cfm holley carburetors is around 0034 0035 inch. Plus it had an electric choke. This was using a holley 4150 carb 870 cfm vs avenger which is very similar to the 3310 that i have which is a 4160 holley. I will start with the engine cold and bring the rpm to around 2000.
The restrictors in this carb had been drilled to a larger 0055 inch. The hot rod series offered several advantages including four corner idle adjustment screw in air bleeds and idle feed restrictors thatd make tuning easier. The idle air bleed will also affect the amount of fuel. I usually start with a baseline calibration set the mixture screws about 1 14 turns out.
Gallery of Holley Idle Feed Restrictor Tuning
