I have a Holley Street Avenger 670cfm that I got used. I was wondering what goes into the top hole in the picture? Im pretty sure that a mixture screw gose in the bottom one, is this correct?

The two holes are on the drivers side behind where the throttle hooks up.
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So this is the secondary side (back) metering block? Are you sure these holes are even drilled out, or just blanked? The bottom hole would be for the idle mixture screw, and some Holley carbs do have four hole idle circuits, but that's pretty rare. The upper hole appears to be blanked off as well. On some carbs designed for supercharged applications there is a hose fitting here on the metering block to provide a reference pressure to the power valve.

 

Now that I look at it closer the top one dose look like a blank. This carb has three idle mixture screws in it already.Is this normal or dose it mean it should have four?

 

It should have either two or four, depending on how the carb is configured. Sounds like you need another one.

 

Alright thank you Joe.