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Extra mixture adjusting screw on Holley 1920?
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Author:  Eric W [ Fri May 16, 2008 10:44 pm ]
Post subject:  Extra mixture adjusting screw on Holley 1920?
I recently cobbled together a spare 1920 just to have an extra around if needed. I noticed an extra brass screw, located just behind the float bowl that goes into the throat, into a hole next to where the accelorator pump shot comes out. A hole to the idle circit?. I threw it on to see how it ran. Not bad, much like the one I had. A few things I noticed though. If I turn the screw out (using a small screw driver for glasses), the carb leans way out and the engine will die. Turn it in and it smooths out then gets the pfft pfft pfft from the exhaust. There also seems to be a flat spot when accelorating, though at freeway speeds it pulls pretty good (well, for a 3600lb car). I have no clue what the numbers are on the carb (there are some, just don't have them with me now), but noticed that the 1920 I was running had the same screw, except there was lead covering the hole and the screw could not be turned. What exactly is this, and is it part of adjusting the idle mixture? My shop manual makes no mention of this screw.
Author:  HyperValiant [ Sat May 17, 2008 6:44 am ]
Post subject: 
Eric,I am curious too as the 1920 I put together for my Daughters car has this screw also.
HyperValiant
Author:  SlantSixDan [ Sat May 17, 2008 6:51 am ]
Post subject: 
Show us a picture? Starting in the late '60s, Carter and Holley carbs started being produced with adjustable idle air bypasses to allow more precise dial-in of the carburetor's mixture at idle and low speed. This was necessary once emission regulations began to tighten up, and went hand in hand with needle screw modifications that limited the range of adjustability by whoever might happen to open the hood. The idle air bypass adjustment was made on the flow bench at the time of final carburetor calibration as one of the last steps in manufacture; it was not intended for field adjustment, so it was hidden behind a plug of one sort or another. It sounds like the plug on the one you found has gone missing. The green arrows in the pic below show the plug hiding the air bypass adjust screw on a '69+ Carter BBS:

Image
Author:  Aggressive Ted [ Sat May 17, 2008 10:06 am ]
Post subject: 
Eric,

I picked up another 1920 Holley that has that screw also. It is nice for adjusting the idle air bleed instead of drilling. You can dial out the flat spot. I wish I had one on my Ecnomaster that "Argentine Juan" has been helping me with. I have to drill it out to get the right air bleed size. The hole in the Economaster carb throat is potted over with lead. I am taking it off this weekend.......good weather finally to do that. It doesn't take much air to get the emulsion circuit working.

How many turns out is your idle mixture screw at the throttle blade location?
How many turns out on the idle air bleed in the neck? You only need about a #60 drill size hole to make the circuit work.
Author:  Eric W [ Sat May 17, 2008 10:27 pm ]
Post subject: 
Dan, sorry, no digital camara. Ted, I turned my idle mix screw out about 1.5 turns or so. The idle bleed screw, well, I just adjusted it until the engine smoothed out and I got the highest vacuum reading (only 17", not much), so I don't know exatly how much.
Author:  SlantSixDan [ Sun May 18, 2008 9:06 am ]
Post subject: 
No camera at Eric's. OK, can anyone else shoot a pic of this screw on a 1920? I can't bring to mind where it might be...
Author:  Aggressive Ted [ Sun May 18, 2008 10:28 pm ]
Post subject: 
Yes, I can take a picture and email it to you Dan. It adjusts the idle air bleed at the very top inlet area. Dan PM me your email.

Bye the way I was telling you about my Economaster idle air bleed being plugged with lead. Argentina Juan wanted me to drill it out with a #65 drill.
Well today I had it off and drilled it out. What a difference!!!!!! It went from a 750 rpm idle to a 400 rpm idle just drilling it out. I can actually screw the mixture screw back in to a normal setting. I had it out 5.5 turns, now it is about 2.5 turns. Still need to take it on a drive and dial it in. I can sure hear the idle circuit working now.

That should help with the mileage :!: :D This will be a good test week.
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