The Stovebolt Site Forums The Shop Area Engine Shop Dual Carb Fast Idle

About 5 years ago, I installed an Offenhauser dual carb manifold with Carter YF's on the 235 in the '51. Everything works great except...I have no fast idle when it's cold. They are auto choke carbs, and the fast idle doesn't have enough strength to overcome the return springs. I have considered using the original throttle cable, but due to the nature of the linkage, I can't figure out a good way to do it. I'm sure anyone with a dual carb setup has crossed this bridge. What worked for you? Right now, I wedge a piece of wood between the gas pedal and the brake. There has to be a better way. Thanks in advance!

I don't understand the problem???

With auto choke carbs, the engine should be started by pushing the footfeed to the floor and RELEASING. This should set the fast idle, the return springs have nothing to do with the operation of the fast idle.

You might have to pump the footfeed a couple of times, but once this is done, REMOVE foot from footfeed.

Jon

Jon-

Thank you for your reply.

The return springs are pulling against the fast idle. The fast idle doesn't have enough strength to overcome the springs.

Allow me to clarify a couple of things. First, only the front carb is choked. Second, upon the recommendation of a knowledgeable, long time member of another forum, I used three return springs. The original, and one for each carb. Too much return spring tension?

Tim - I still am not understanding the issue.

Assuming the choke mechanism, and fast idle mechanism on the carb is not worn out, and is properly adjusted:

(1) Pushing the footfeed to the floor should allow the autochoke spring to almost close (the unloader circuit should cause the choke plate to be slightly (without knowing what carburetor by number, guessing 3/16 inch) open.
(2) Now releasing the footfeed should allow the choke plate to completely close assuming ambient at 65 degrees F. or less.
(3) When the choke plate completely closes, the fast idle link should go to the highest step on the fast idle cam. The throttle adjustment screw should now be maybe an 1/8 inch off the stop, and Hulk Hogan should not be able to close the throttle (well, maybe Hulk could , but certainly not a throttle return spring (or 3) ).
(4) As the engine warms, hot air should allow the choke spring to relax, and the internal choke pull off to begin to open the choke plate, allowing the fast idle cam to rotate and the throttle return spring(s) to begin to close the throttle, and slow the idle.
(5) Having one's foot on the footfeed during the warm-up process will DEFEAT the action of the fast idle cam.
(6) The fast idle link, over time, will bend and thus need adjusting.

Where in central Missouri (in other words, how far from Eldon) are you located?

And speculating here, is your friend a Mopar enthusiast??? For some reason, known only to an unknown ambulance chaser, many Mopar products in this time period actually used the throttle return spring to close the throttle. If you were to place a carburetor for a Mopar on your workbench unattached to anything, the throttle plate would normally be approximately 60 percent open!!!! Conversely, placing the Chevrolet carburetor on your workbench unattached to anything would show the throttle plate closed. In other words, if the throttle return spring on a Mopar were to break while the vehicle was being driven, the throttle would immediately go wide open. The first time it happens can require a change of clothing!

Just for the record, I have a single throttle return spring on my shop truck with the 450 HP 390 utilizing two Carter AFB 4-barrel carbs which run simultaneously.

Jon.

Jon, I remember an incident in Dad's shop in the mid-1950's where a late-1940's Plymouth had a piece of throttle linkage fall off, and went almost wide open in one of the service bays. I believe part of the accelerator pump system was responsible for that partial throttle opening when it was unopposed by the return spring, right?
Jerry

Jerry - you are correct. The accelerator pump spring on Chrysler carbs was assembled so the pump was normally down. The throttle return spring caused the pump to lift, charging the pump spring. Still have never heard an explanation.

Jon.

Jon-

The carbs are numbered 2101S.

When time allows, I'll dig into the front carb to make sure everything is as it should be. It seems like it worked when I used it as a single.

The multiple spring advice came from a fellow Chevy owner, although he had wide and varied taste in cars, especially European.

BTW, I'm located in Rolla. I bought a pair of your kits when I did the dual setup. Don't you remember? ;-)

Tim - at my age, I tie a string around my finger to remember something. Then I write a note to remind me what the string is for. Then I lose the note!!!

I do believe three springs are probably overkill.

Jon.

I was told that the combination of size of the pump orfice and the pre-loaded spring resulted in a slow, steady pump shot that didn't vary even if the throttle was opened very rapidly. Sort of made sense to me, in a weird way, but it still didn't overcome the tendency to run away if a return spring broke or the linkage came unglued!
Jerry

Jon -

Just to update you on my carbs. It seems like the fast idle cam wasn't instated quite right. Also, it appears the fast idle link wasn't bent right. I have reset the link as well as the choke and everything seems to be working properly now, even with the extra springs. Thank you for the advice!

Tim - glad you found the problem.

Jon.