Holley 4779 with weird iab

[1969Camaro]

This carb started life at a big rebuilder and then I think another person did some drilling.

The new down stream IAB exits in the Venturi via the new hole drilled and then epoxy covered.

Why??

[1969Camaro]

Main body side

[rgalajda]

Used only with auxiliary idle air bleed. # 5 on picture below.

I can only assume this was a trial and error experiment someone tried on the carb you pictured.

Some of the OEM Chevrolet Holley carbs had a lower idle air bleed drilled in the main venturi ( similar to quadrajet carbs ) The 1969 427/425 HP Holley carb had this. Idle air fuel ratio was in the 14.5:1 range. I have experimented with this on my car. Since my cam has more duration and tighter LSA , this made the idle air fuel ratio too lean for it to idle .

As noted #5 in the picture . I have yet to see a holley carb with this passage used. So I don't know the application .

[1969Camaro]

Why would someone do this and what is the advantage? Btw, the idle feed restrictors are in the upper position.

[rgalajda]

For a mild cam engine this could correct rich AFR in the low throttle opening if you drilled the right size. ??????

Same reason the cut the air horn/choke off . They thought better air flow. Generally it does not help.