Plasticman

I am running dual 500 CFM Carter Comp. AFB's on an Offenhauser manifold on a "stout" 355. Progressive linkage (rear carb acts as primary, and then front carb kicks in as sec.). Have set idle by closing the front (sec. carb.) throttle plates and idle mix screws. Adjust idle by rear carb plates and mix screws only. Lar's BG Demon article got me thinking maybe I should try to adjust idle using both front & rear carbs (for a better quality idle). Really have no running issues other than it does run fat all the time (will get to those items later when the season is here - found a Carter Strip Kit, and got the Edelbrock metering rod springs to play with). But can't complain since with no chokes, it still fires up nicely and runs real good.

My thinking is that with the Offy manifold and running off the rear carb only, there could be significant fuel distribution problems that could be cured by using the front carb for idle as well.

Comments/suggestions always welcome.

Plasticman

plaidside

Is the Offy a single or dual plane manifold? I had not luck with single plane manifolds on the street. Low idle vacuum and low speed performance.
For best idle quality and drivability, your best bet is a dual plane manifold.
The way to tell is look at the carb mounting, is it separated or one big open hole.
I have never seen a factory hi-perf engine with a single plane manifold.

magicv8

Many years ago, I ran dual AFBs - and the setup went something like (40 year memory) this. Warm engine. Close all idle fuel screws, and set the idle air screws to where the plates touch the throttle body. Then turn the idle air screws in 1 turn, idle jet screws out 1 turn. Start the engine, and if the idle is not high enough add to each of the air screws in half turn increments until the car can idle. Back out the idle fuel screws as evenly as possible until the idle is even. I am assuming that the car is currently running with the two active idle fuel screws out about 2 to 2.5 turns. Run a very light choke on both carbs for cold weather starting (back then I lived in Buffalo and drove the car everywhere), since two pumps will keep it alive until it warms up. Set the pumps in the minimum movement holes (farthest away from the pivot).

The carbs can be set with a vacuum guage or air meter setup, but 40 years ago - no one I knew had them. :skep: (Well, okay - Chevy dealers set Corvairs with them.) :p:

JohnZ

I've had several C1 2x4's (with stock WCFB's, not AFB's), and both carbs function at idle and need the idle mixture screws adjusted. Off-idle, they run only on the rear (primary) carb until the progressive linkage opens the front (secondary) carb; the rear carb was intentionally jetted on the rich side to make up for the lousy mixture distribution to the front cylinders, so all of them "ran rich" during normal operation. The front carb has no idle speed screw - a tang on the linkage lever bottoms out on the carb base instead so only the idle mixture ports are exposed to manifold vacuum until the progressive linkage pulls on the lever. The front carb is always operating in an idle mixture mode, using a small amount of fuel, so the fuel in the float bowl doesn't go stale if the driver never opens the front carb. :thumbs:

Plasticman

Plaidside,

Manifold is a single plane. Understand fully about not working too well at lower RPM (in fact I just got through replacing the single plane Torker from my 51 Chev. P/U for that very reason). But this setup is not really a low RPM engine, and runs pretty good for how I use it. Just wanted to know what is the Forum's resident experts' ideas on setting up for best idle. Thanks!

Magic,

Thanks for info. I always use a vacuum guage when setting idle, and find that this Vette's idle is around 15 inches at 1K RPM (not bad for the cam being used). Just think that using both carb's for idle might be a better way of setting the idle. No problem running cold, even without any chokes (The Carter Competition AFB's don't have chokes - Vette came with them when I purchased it), which certainly indicates a overly rich setup.

JohnZ,

Thanks for insight on WCFB setup. Will try setting idle with both carbs. Will be an interesting exercise, if nothing else.

Like I said earlier, the Vette runs just fine except for being on the rich side (otherwise I would of done something long before this), but always looking for a "better way"! That is where the fun is!

Thanks guys,
Plasticman