Just to make sure people are on the same page when modding their carbs:
In Holleys they have two distinct circuits that have an affect on certain functions at certain times:

1) Idle circuit- Mixture is determined by the idle air bleed, closing the bleed lessens the air in the circuit making it richer. Depending on model of carb the idle mix screw typically will change the amount of fuel delivered to the idle mix...on some emissions carbs however the screw only changes the amount of air/fule being delivered.
2) Transition circuit- This is "off idle" or foot off the brake and starting to drive and/or very low speed low demand driving. This mix is detemined by the idle circuit and is bigger than the idle port near the base plate of the carb. It is usually a slot and is brought on line by the throttle plate opening and sweeping by the slot on greater demand. Also during this time to keep the engine from stumbling on the way to the main circuits some extra fuel is added by the accelerator pump if more pedal is applied.

Compared to the main circuit these two circuits are like a squirtgun adding water to a garden hose, although they contribute, they don't affect your mpg unless you are putting around town at very low rpm and never get into the mains.

3) Main Circuit - 99.8% fuel for this circuit is metered by the main jet(s) in the carb so if our highway mpg is bad and you have traced it back to the fuel delivery it may need a leaner jet. These come on when the throttle plate is past the transition slot and fuel is being pulled from the bowl through the jet and into the carb through the venturi and "booster" in the throat by vacuum. "When" this occurs is determined by the 'high speed" air bleed (typically in the carb throat near the idle air bleed).
The factory calibrated this bleed based on the demand of the vehicle...smaller diameter = mains come on sooner, and larger diameter = mains come on later...typically the mains come "on" in he 1800-2200 rpm range depending on the vehicle (Obviously a truck being heavy will demand more throttle plate to get it going and thus the bleed may be small so the truck doesn't get horrid gas mileage as the carb goes right to mains when leaving a stop light. Whereas a lighter vehicle with performance gearing may not have enough 'demand' that it literally can 'idle' around town and never activate the mains until doing 70 on the highway).

If your vehicle is no longer "stock" these improvements will help in getting better idle adjustment and quality, or better 'off-idle' performance, and/or better mpg assuming the carb is in good working order and everything else is good to go. Unfortunately since we aren't totally talking about a 4 barrel on a chevy, you will need to blaze your trail through trial and error to get the correct 'new' settings...remember to only change one thing at a time or you might get hung in a position where you won't know what to 'undo' when something undesireable happens.

Just wanted to add some detail into the process to think about while you guys are tinkering with your carb settings.

-D.Idiot

rooster,
I would tune it for best drive-ability for your truck. I have found that the smaller the main jet is with a large idle air bleed the more hesitation I got off idle during acceleration. So the transition from off idle was very frustrating and worsened the hotter the temperature got. The smaller idle air bleed seems to work much better to reduce this effect. So trial an error. If you jump up from the 612 up to a 64 you will notice the transition hiccup is not as bad, so the richer main jet masks the problem, but results in poor mileage. To get good mileage with a smaller jet you will need to make the air bleed smaller as well.

FYI, I am currently running a #56 jet with no hiccup during the off idle acceleration transition.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Thanks D.Idiot and Ted for the replies. Think ill downsize the bleed to where yours is Ted and jet down and see if I can get it to where I want it.
My carb has the brass insert tube unmolested in any way. Think I can use the ball and punch method to peen the top of the tube smaller or should I solder it up and redrill?

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if it has breasts,wheels,or freon it requires constant attention.

ok i changed the jet to a 59 (all i had) from a 612. instantly seemed to have more all around power and better drivebility, so im thinkin i may need to go lower on the jet yet to get on the leaner side.

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1965 Plymouth A/FX 512ci (under construction)
1968 Dodge Dart SL6 14.25 @ 91.5 1.91 60'
1973 Dodge Dart SL6 Stock
Killer Bee's Member Since 09'
http://www.forabodiesonly.com/mopar/sho ... p?t=128702
http://www.youtube.com/user/805moparkid

Any idea what happened to the pics. from this old thread? I'm having idle issues with 1920 holley

Time moves on, so do users and the account/webspace they link the pics from. We don't have onsite picture storage on this board...