Tweet
I want to adjust the idle speed on my 57 dual quad setup, but none of the manuals I have tell you how to do it. They only say to "adjust idle speed screw". Since each carb has an idle screw and mixture screws, it only makes sense that both need to be set. The specs I have call for 800-850 rpm's. How do I accomplish this using both idle screws?
-
-
Re: C1 Carb Adjusting
Check the owners manual.
Rob -
Re: C1 Carb Adjusting
According to the owners manual, the 'throttle valves of the front carberator are to be fully closed at idle'. Am I to assume that the idle screw on the front carb is just for looks only? I am surprised that the owners manual has this and not my other manuals. Thanks for the help Rob.Comment
-
Re: C1 Carb Adjusting-Mike
Mike, A quick pass on this, The two air idle screws on the front of each WCFB's are generally djusted wih avacuum guage atached to the base of the rear carb. Object to adjust to the highest engine vacuum. Usually his sdjustment increases the idle RPM which is adjusted (downwardif necessary) by the idle adjustment screw on thelinkage side of each WCFB, In theory the front carb did not need or use an idle adjustment screw. Theory out the window,owners installed an idle adjustment screw there as well. I adjust mine so that it baely increases the RPM. The rear WCFB is most if not all responsible for tweeking to the desired RPM. egards.Comment
-
Re: C1 Carb Adjusting-Mike
Thanks for the info Jim!Comment
-
Re: C1 Carb Adjusting-Mike
If you have the "early" carbs (used on all 56's and early 57's), they have "idle air screws" on the driver's side of the base, which originally provided idle speed adjustment through an internal air bypass circuit; with this setup, all throttle valves were tightly closed at idle. A Service Letter instructed the dealers to close them entirely on both carbs, to add an idle speed screw on the linkage arm on the rear carburetor, and to use the idle mixture screws on the front of BOTH of the carb bases to adjust idle mixture with a vacuum gauge, and to use the added linkage screw on the rear carb to adjust idle speed. The front (secondary) carb is part of the idle equation, and needs its mixture screws adjusted too; if its idle system is shut off, fuel sits in the bowls and gums up the metering passages unless you get on it occasionally to flow fuel through it.
This procedure is shown in an illustrated "Corvette Engine Tune-Up, 1956-57" booklet, originally issued to dealers by Chevrolet in August, 1957; I got mine from Danchuk about eight years ago under their item #70150. A good companion manual from the same era is the "Servicing The 1957 Corvette Engines And RPO Equipment" manual, also issued by Chevrolet in 1957, with 72 illustrated pages on Engine Tuning, servicing Heavy Duty Brakes and Suspension, T-10 Transmission Rebuild, Positraction Rebuild, and 20 pages of Fuel Injection tuning and rebuild information. Terrific 1956-57 resource covering many systems not covered in the ST-12 or Passenger Car Shop Manual. Also from Danchuk, their item #V70151, also available from the NCRS Store.
While you're at it, get the "Corvette Service Operations - Chassis" and "Corvette Service Operations - Body" manuals from the NCRS Store, and you'll have every GM Manual ever printed for the 1957 Corvette.
You can't have too many manuals!Comment
-
this setup is now the hot setup
with barry grant carbs,using a central air bleed to set the idle speed. using the butterflys to set the idle speed caused problems with the idle transfer slots being opened too far with some types of camshafts and then the idle mixture screws have no effect because you are now idling off of the the idle transfer slots not the idle ports which are controled by the idle mixture screws. with the barry grant carbs you keep the idle transfer slots at .020 opening which is ideal and use the air bleed screw to change the idle speed of the engine. this just proves that all that is new may not be that new. most older WCFB carbs in the ealy 50s used this air bleed idle speed control system. some pontiac AFB carbs used the air bleed idle speed setup alsoComment
Comment