My Carburetor

 My carburetor is a 600 double pumper. I modified it for blowthrough and went further by tapping and threading all passages for more fine tuning.

This picture shows my primary metering block. I installed  2, 6/32" brass set screws which I can drill out to any size with micro drill bits. These orifices are fixed from the factory on most standard Holley's. These are the idle feed restrictors. I reduced mine from the factory .031" down to .026" for a nice and lean off idle cruise. The ifr's work with the idle air bleeds to create the idle and off idle air fuel mixture. Bigger= more fuel,smaller= less fuel.

A closer shot.

The above shows me drilling out the factory air bleeds. I drill and tapped for 10/32" brass set screws. I can drill out with micro drill bits for tuning. These mods are rarely needed for a naturally aspirated engine with a properly sized carburetor.

Quickfuel explains the airbleeds operation well. My primary idle air bleeds are .0625 and the high speed airbleeds are .020. The secondary idle airbleeds are .028 and the high speed airbleeds are blocked for now. I am still playing around with the tune.

I have drilled and tapped the power valve channel restrictions for an 8/32" setscrew. The pvcr's give additional fuel flow when the power valve opens.  My idle vacuum is about 13" and  the powervalve I chose, opens at 10.5. A small primary jet helps create a lean cruise but then at w.o.t may be too lean. I am able to use a small primary jet with a larger pvcr. This allows a lean cruise with a properly rich w.o.t.  My primary jet is a 68 with a .070 pvcr. With this combination I have 11.8-12.3 afr wide open throttle on just the primary side, with the secondaries disconnected and wired shut. My cruise afr at 2,000 rpms are 13.5-14.5. Now that I know the primary side of the carb is properly tuned, I can connect the secondaries and continue tuning.