Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
idle help
Cliff Ruggles:
Thanks for the pics, as with anything else a picture is worth a 1000 words.
For the readers, using a tiny direct main bleed/blocking the indirect bleed and relocated the upper IAB's to the airhorn would pull a LOT of fuel from the idle and main system. Haven't tried that and woln't be in one of those but I be assured it will fatten it up everyplace and no doubt it would be fine with .031 idle tubes and .046" DCR's at 14" vacuum at idle speed, especially if you didn't open up the LIAB in the truck big block carb.
Those carbs were pretty "fat" without any modifications and why Edelbrock patterned their "850" cfm model 1910's after them. In stock form with a .038" idle tube and .055" DCR's the 1910's have enough idle fuel for a Pro-Stock dragster engine!.........Probably why a lot of customers call up here bitching about them being a little "fat" at idle on their "mild" street engine build......FWIW......Cliff
77cruiser:
If that thumb wasn't in the pic I'd swear that was my carb. ;)
If you have time & an extra core try one.
Cliff Ruggles:
PLENTY of spare cores, no time and no need to go that direction. My personal carb is custom tuned using one of the "recipes" in my book. Nothing will outrun it, and believe me the dyno shop I use has tried, and I've ran every size and type of carb out there against it at the track and it's always posted quickest ET and highest MPH.
For certain applications I will relocate the upper idle bleed to the airhorn and use a small size and block the ones in the main casting above the DCR's.
The factory actually did this on a few of the "high performance" units. Coincidentally was working on one of them today from a 1970 Ford 429 Cobra Jet engine. Talk about a "special" carb. It had tiny MAB's and about the smallest upper IAB's you will find on any Q-jet in the airhorn. They worked well for what they were designed for as the 429 CJ engines were heavy breathers and poor signal to the carb at idle, off idle and low RPM's..........
73ss:
other than getting away from large idle tubes and dcr's, what are the benefits of this modification?
Cliff Ruggles:
Basically blocking the indirect upper IAB in the main casting and moving it to the airhorn requires a much smaller size to be used.
Keep in mind that an upper bleed in one location or the other is required or the fuel bowl will syphon down after the engine is shut down.
When the throttle plates are closed and engine is idling there is high vacuum or strong "pull" at the mixture screw holes and any exposed transfer slot.
For most stock engines the signal is very strong and fuel pulled easily into the engine. The factory used a lower idle airbleed just above the throttle plates as a vent and to reduce the signal some, and other upper airbleed to reduce it even more and provide a vent to prevent syphoning.
The sizes of these holes vary considerably depending on carburetor part number and application. It is part of the total calibration since it not only provides idle fuel it also provides fuel thru transition to the main system as more transfer slot is exposed.
Once in a while I'll venture in that direction with a carb I'm building here. When I do it's typically when the engine "builder" has went with a pretty "hefty" can on a tight LSA and the compression ratio isn't going to be high enough to provide good signal at idle speed. Experience has taught me when to do this and we almost always increase base timing at the same time, so I may have the owner send the distributor here as well so I can shorted up the curve some.......Cliff
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