Carter 3721 AFB Fast Idle Cam

[Con X.]

After a failed first attempt to start my rebuilt engine because of a bad fuel pump, I'm rebuilding the entire fuel system, including rebuilding the Carb. Using both the CSM and Carter rebuild pubs this time. Carter said inspect the fast idle cam and the "steps" on the cam appear worn. Trying to find a new one, but difficult.

How important are the steps? Other forums seem indifferent. Thinking I can simply file gently to improve the steps and the screw will compensate.
Appreciate any help.

[Duke W.]

Re: Carter 3721 AFB Fast Idle Cam

I don't think the "steps" are critical, but the cam should have decreasing radius as it backs off, which is what reduces idle speed. Be sure to bend the links as required so the fast idle screw centers on the index mark when the choke valve is fully closed, then set the fast idle speed properly, and this is done with the engine fully warmed up.

For example the 1963 Corvette Shop Manual specifies the fast idle speed for my 340 HP 3461S as "1750 Hot on Car". This means that you set the fast idle speed with the engine at full operating temperature. To do this open the throttle slightly and manually set the fast idle cam so the fast idle screw is on the index mark, the set fast idle speed to spec with the fast idle adjustment screw and slightly open the throttle to let the cam relax.

On a cold start at cool to mild temperatures the fast idle speed within a few seconds of start up will stabilize at 1500. If you don't touch the throttle the engine speed will increase to about 1750 by the time engine temp reaches about 100F or a little over.

If you drive away as the engine warms, the choke valve tension is slowly released, which allows the fast idle cam an opportunity to retract every time you open the throttle until the cam is fully retracted and the engine is down to normal warmed up curb idle speed. As long as this action is relatively smooth and doesn't cause any drivability problems the fast idle cam is okay. The contour and steps don't have to be super precise.

Use the proper GM service manuals as your primary source for all the carb adjustments. For the 3721S/SA/SB that would be either the 1964 or 1965 Corvette Shop Manual Supplements with base manual being the 1963 Corvette Shop Manual. Don't leave home without them!

Duke

[Con X.]

Re: Carter 3721 AFB Fast Idle Cam

Appreciate all the info. Using a set of nautical dividers I confirmed the cam radius decreases so I'm not going to file or change it. Thanks.

The 64 supplement only addresses the Holley 4150 used on the 365hp engines, and I have a Carter 3721SB so I'm following the 63 CSM. The Carter documents sometimes add additional detail but the procedures mirror the CSM. Still waiting for new fuel filters, so hopefully try to start again next week.