Posted 4/3/2022 by admin

Holley Idle Transfer Slots

Holley’s Ultra models have an adjustable idle air bypass screw (yellow arrow). Air for the bypass goes through the holes indicated by the blue arrows. To supply the required amount of idle air without using up all the transition slot, it may be necessary to drill holes into the butterflies. Unfortunately, when the curb idle adjustment is opened past the stock setting, the throttle blades uncover the transfer slots. These slots are intended to pull additional fuel from the carburetor's. The adjustment is made using the Idle speed Screw on the side where the Throttle linkage hooks up. Turn the Carb over and slowly back off the Idle screw until the Transfer slot BELOW the Throttle Plate looks like a Square. You do need to be sure the Choke fast Idle screw is off and not holding the Throttle plate open. By Joe-JDC - 3 Years Ago.

Holley Idle Transfer Slot Restrictor
Holley Idle Transfer Slot Adjustment

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PostPosted: Thu Apr 10, 2008 8:32 pm

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Board Sponsor

Joined: Tue Oct 29, 2002 8:27 pm
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Location: Salem, OR
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so noone would be forced to use a BBS again

I just took apart a BBS and the Bendix/Stromberg carb and I can see where these have the advantage over the holleys (the BBS is fully disassemblable and has no blind passages/sealed metering block)... The stromberg is even more simple and it looks like it was designed to be able to knock the main venturi out and change it and increase the CFM rating with no major hassle... too bad there's not a kit with these major assembly parts in it...
LOL...
-D.Idiot

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PostPosted: Fri Apr 11, 2008 2:06 pm

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Supercharged

Joined: Mon Jan 15, 2007 5:05 pm
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Location: Black Diamond, WA
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SlantSixDan wrote:
What-all kinds of info are we after for background/carb selection?

Juan wrote:
bleeds/jets sizes according to years, accel pump discharge hole size. venturi and throat sizes, etc. also P/N for chrysler specific applications.

SL6Dan,
Could you help us out?
Thank you for looking up the specs.

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PostPosted: Tue Apr 15, 2008 5:10 am

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Joined: Mon Jan 12, 2004 4:20 am
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Location: Argentina
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so noone would be forced to use a BBS again

holley 1920 ain't the best one barrel around, is just the one maker that casted one barrels for more time and the parts avalibility kicks ass around the world. Go try'n find a needle for any carter carburetor outside the states and say 3 or 4 more countries: you're screwed up! the stromberg carb is WAY better than BBS's and Holleys. Now that I think about it, I've seen a lot of US made 1 barrel holleys with the detachable steel throttle base (that will be like you say about the BBS, fully accesible passages and such)
Anyway ain't a big dealie to pop some plugs, clean and repop close on a 1920... I'm amazed at how guys wich such an experience in fiddling with way more complicated mechanich issues frown their foreheads when it's time for poppin' and cleanin metering blocks and carbs.

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PostPosted: Tue Apr 15, 2008 9:41 am

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Supercharged

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PostPosted: Tue Apr 15, 2008 10:41 am

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Turbo EFI

Joined: Sun Nov 03, 2002 1:49 pm
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If possible, it would be nice to see some pictures of these modifications on the 1920's. You know, what plugs to pull, what to do once they are out, etc.

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I agree. It would be nice to See some of this. Guys You are doing a Great Job with this thread. THANKS
Frank

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PostPosted: Mon May 19, 2008 9:48 am

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Supercharged

Joined: Mon Jan 15, 2007 5:05 pm
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Location: Black Diamond, WA
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Juan,
I had the Economaster off this past weekend and drilled the potted over idle bleed hole with a #65 drill bit.
When I fired it up my normal 750 rpm idle went to 400 rpm. Now you can hear the transfer slot working very easily. It sounds like a huge water large water fall in there. I was running with the idle mixture screw at 5.5 turns and I roughly set it at 2.5 turns. It sounds and feels good cold no bog, but after driving 70 miles this morning to work, it was very warm and acted very differently. It feels like a hicup or very slight bog. It may still be too rich. I am thinking I need to open the hole up a little more? or lean out the idle mixture screw some more.
I am not quite sure yet. The throttle is way more responsive now and hard to hold still. Any slight pressure change on the gas pedal makes the car respond right away. I am having to learn how to drive all over again. That little change made a huge difference in the way the engine responds.
I will play with it on the way home tonight to see what it wants.

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PostPosted: Mon May 19, 2008 10:23 am

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H'mmmm. You may want to try drilling a vacuum calibration hole:
This hole is for calibration of the power valve and vacuum advance. Some carburetors have it and others don't, but once it's there if you don't want it any more it's easy to plug...

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PostPosted: Mon May 19, 2008 10:32 am

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Turbo EFI

Joined: Sun Nov 03, 2002 1:49 pm
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Location: Lubbock, TX
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Ted, I've found that turning that little screw even a little has big effects on idle and throttle response. My engine still has kind of an uneven idle, but it seems like it wants to run all day at 70+ with no problems. It also pings slightly on steep hills now...though it dosn't on long freeway grades at speed. Maybe time to futz around with the distributer springs again....

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Supercharged

Joined: Mon Jan 15, 2007 5:05 pm
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Location: Black Diamond, WA
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Eric,
I have noticed the same thing that the idle air bleed has a huge! effect on idle quality. This Economaster carb I have (12R-7610 for 1968-72) has never had a normal idle mixture screw setting. It has always been very fat 5 or more turns to get it to idle smoothly because the idle air bleed was potted over.
Opening it up with the #65 drill really kicked the emulsion circuit in gear.
That little brass screw your adjusting is the idle air bleed adjuster. I am not sure yet where it should be set. All I know is that by opening it up some with the .035 #65 drill has made it run richer. Need to figure out the next tuning steps...............
I will take a picture tonight of the brass adjuster and send it to Dan.
What size main jet are you using now?

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PostPosted: Tue May 20, 2008 1:03 am

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Turbo EFI

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What size main jet are you using now?

A #58.

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Supercharged

Joined: Mon Jan 15, 2007 5:05 pm
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Eric,
I sent the pics to Dan last night. He should give us a heads up soon on the brass idle air bleed screw. On all the 1920 carbs I have, there seems to be a large well at the top, but in all reality you can't push any wire or drill bit in the idle air bleed location. It seems that Holley has plugged them off.
Now that I have opened up the idle air bleed Juan is suggesting that I try going up two sizes on the jet to balance everything out. If it isn't raining too hard tonight, I will pop in a #60 and see what happens.

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PostPosted: Tue May 20, 2008 9:47 am

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Yup, I definitely think this screw you're showing me is the Holley 1920 version of the Carter BBS factory-adjustable idle air bypass, not originally intended for field adjustment and so plugged off after adjustment on the production line.

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PostPosted: Tue May 20, 2008 12:17 pm

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Supercharged

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Dan,

Do you have any books with specs on what this adjustment should be

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PostPosted: Tue May 20, 2008 2:38 pm

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Do you have any books with specs on what this adjustment should be

It's not a question of specs. This screw was adjusted 'live' (dynamically) on a flow bench at the end of the carburetor assembly line. It was not intended for field adjustment once the carb was built and out the door.

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Transition slot exposure is critical because it affects both idle quality, idle air / fuel mixture as well as 'tip-in' or 'off-idle' response.
Too little or no transition slot exposure on the primary side on most engines will cause a stumble or hesitation on tip-in. Viewed with the butterflies in the idle position, base inverted.
Click on picture to enlarge Holley

Too much transition slot exposure on the primary side on most engines will cause: a rich idle / exhaust smell; little to no sensitivity to idle mixture screw adjustment; soft tip-in stumble or hesitation on tip-in or part throttle response.
Click on picture to enlarge
The following picture shows a near perfect transition slot exposure for most engines. The shape of the transition slot exposed (under the butterfly) will look something like a square.

Holley Idle Transfer Slot Restrictor

Click on picture to enlarge
To adjust secondary transition slot exposure a secondary idle screw / throttle stop is provided. On TMP Carbs' preppared carbs, the idle speed screw is accessible from the top. To access it, with engine off, simply open the throttle to expose the secondary idle speed screw and turn.

Click on picture to enlarge

Idle Fuel Restrictions (IFRs) or Idle Jets
Replacement of 'idle fuel restrictions' (IFRs) also referred to as 'idle jets'.
IFR's (idle jets) on TMP Carbs' prepared Holley carburetors utilizing Holley metering blocks are located as indicated in the diagonal passage intersecting the idle well providing communication to the idle passage in the metering block face. TMP uses 6x32 brass set screws for the IFRs.

Here is a another view showing the angle of incident and location of the IFRs.

Power Valve Tuning

Holley Idle Transfer Slot Adjustment

Power valve tuning is critical for best mileage, throttle response and performance. By far, most high performance and racing engines require a late opening, low value power valve such as a 2.5 or 3.5. The proper power valve value can be determined by measuring manifold vacuum when accelerating from a dead stop or low speed taking note of the vacuum reading as the secondary barrels begin to open.

TMP Carbs article on: http://www.detroitdragway.com/tuning-in-photos/