The top cause of carburetor troubles is the overexposure of the transfer slots at the idle condition. On the right yellow arrow is the replaceable style for the ultra range of holley carbs.
Tuning Horsepower Innovations E85 Carburetors
Idle feed restrictor tuning. Idle feed restrictors circled limit the total amount of fuel available to the idle and transfer slot circuits. This will give an important clue for your tuning. While this sounded bad. 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. When increasing the idle speed of a four barrel modular carburetor adjust both the primary and secondary throttle blades. As u pull out gently pumping the pedal as necessary watch the afr as cruise speed increases.
For a test only at low speeds light throttle plug the booster feed holes in the main body. If you tune the idle feed restrictors too lean the engine will most likely suffer from an off idle lean stumble as an early indicator that youve gone too far. Could also try a main jet about the same size as the ifr i think. The primary idle feed as you stated is 078 but i cant remember right off hand what size the rear are without getting mine out of storage since i am currently not using it. Out of the box the primary metering block has around 028 for the idle feed restrictor and the rear block has 031 on the idle feed size. Use 10 32 set screws.
On the left red arrow is the fixed type most commonly seen in regular metering blocks. The 1 tuning problem. The restrictors in this carb had been drilled to a larger 0055 inch. Plus it had an electric choke. The standard idle feed restrictor size for most 750 cfm holley carburetors is around 0034 0035 inch. If you go too lean on the idle feed restrictors the engine will surge at cruise and hesitate under light acceleration since most engines dont like to accelerate at lean airfuel ratios.
Holley carb idle feed restrictor size i read an article in muscle machines magazine about how most universal holley carbs have an over rich idle circuit and how at steady low load road speeds the transfer slots are doing the fuel metering. The hot rod series offered several advantages including four corner idle adjustment screw in air bleeds and idle feed restrictors thatd make tuning easier. Idle feed restriction idle jets can be located in different positions for different metering blocks.
Gallery of Idle Feed Restrictor Tuning
