Idle air bypass delete on Jet stage 2 Quadrajet?

[davel]

Guys,

I recently acquired a Jet Stage 2 Quadrajet. It's a 1705XXXX APT model. Before installing it, I pulled the entire carb apart to assess what has been done. I noticed that the wells in the main body that connect the airhorn and baseplate for idle bypass air have been plugged. So it looks like the idle bypass air has been deleted on this thing. There are no holes in the throttle plates BTW.

My question is; why did they do this? And should I drill the plugs out? 

This is a K5 Blazer with standard transmission.  SBC 383 With Howard's roller cam 217/225 @050  110 LSA .470 lift

Here are the plugs:
 

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[davel]

Here are the plugs on each side




Here's the channel


And the base plate with bypass passages

[Sonny]

My understanding is idle air bypass supplements the idle system so the primary throttle blades can remain nearly closed at curb idle. Deleting idle bypass air would require alteration of the idle system in other ways to provide normal idle quality. Cliff will surely have a better explanation of why commercial rebuilders do this.

[Cliff Ruggles]

Commercial builders do this so they can install a "generic" calibration into every carb they build.

The need for idle bypass air varies from engine to engine, based on CID, compression, and cam specs.

Engines with low vacuum at idle will almost always need idle bypass air to keep the throttle angle low and on the idle system. 

I have went thru about a zillion of those carbs at this point, and the first thing I do is remove the lead plugs with a small punch, they set up the idle system exactly for the application........Cliff

[davel]

Thanks for the reply Cliff. Your  book has been invaluable for understanding these carbs.

I do have one question that I haven't seen addressed: Is the primary source of idle bypass air for the engine provided through the PCV valve?  If so, is it effective to modify the airflow through the PCV valve at idle?

[Cliff Ruggles]

I don't factor in bypass air thru the PCV valve.  If I need more for any particular application I provide it thru the excellent system already in place in most Q-jets. 

I see folks drilling throttle plates for bypass air and don't understand why you would do that in lieu of using the bypass air system that's already in place.  It is a very precise vacuum leak and when used correctly can keep the carb well on the idle system, even with pretty big camshafts and low engine vacuum at idle........Cliff