General carb question

[egchewy79]

I have a Holley 600cfm carb and am running into a fast idle situation once warm. The idle control screw is all the way out but still wants to idle around 1500rpm. The throttle lever is all the way forward but I can get idle to drop a bit if I really push it forward. I already have double springs on the throttle arm. What other adjustments can be made to drop the idle a bit?

[J R Jones]

There is more than one thing that could cause your problem, and one basic set-up issue comes to mind. The throttle plates must be concentric to the throttle bores.
Remove the carb and hold the inlet up to the light. Inspect to see if there is any light getting around the throttle plates. (holding the choke open)
If they are not concentric the plates hang-up and do not close 100%.
Loosen the throttle plate to shaft screws, close the throttle and tighten the screws. Retest.
jim

[Dgc333]

Make sure the choke step down is not hanging up. With the throttle fully open it should move freely. Also, is the choke opening all the way? If it is not it will keep the throttle on one of the steps on the step down.

Use some carb cleaner and spray around the carb base, intake manifold and any vacuum lines. If the rpm changes you have found a vacuum leak

[FarmerDave1321]

I just posted a thread earlier with a similar issue, lots of great advice here ...

https://thefactoryfiveforum.com/showthread.php?50882-Carburetor-and-high-idle-(Problem-Solved)

[Mike.Bray]

https://www.imagecoast.com/images/MichaelBray1/458267311511184114829469823322515637707691n.jpg

[rthomas98]

I have a Holley 600cfm carb and am running into a fast idle situation once warm. The idle control screw is all the way out but still wants to idle around 1500rpm. The throttle lever is all the way forward but I can get idle to drop a bit if I really push it forward. I already have double springs on the throttle arm. What other adjustments can be made to drop the idle a bit?

Is it revving to the high idle and just not coming back down when warm? or is warming up and as the idle comes down it just stops at a point? Both issues are symptoms of a misadjusted choke. But need to know which one is happening to help. Either your fast idle screw needs adjustment (revs up and never comes down) or your choke needs the adjustment (comes down and stops at the higer level). See the video below gives you a high level on how to do both adjustments:


https://www.youtube.com/watch?v=4BTiXcMHdXM&t=2s

[Norm B]

I am surprised that nobody has mentioned the possibility of a vacuum leak. In the linked thread in post #4 there are good suggestions on how to check for this issue.

Norm

[egchewy79]

Post #3 mentioned it.
I haven't had chance to mess w/ it yet. The fast idle screw adjustment seems to make a lot of sense. I'll check the choke as well to make sure it's all the way open.

[MB750]

Any time I read posts like this the first thing I think of is if this issue has always been an issue, or was anything was messed with that may have caused it?

Carb tuning is fun to me. Assuming this issue manifested itself over time and wasn't accidentally caused by something you did, I'd start with a warm engine and completely remove the choke. I have a manual choke so I know when it's closed, and I have a mechanical linkage on the throttle and I can see when the throttle mechanisms hit the stop on the carb throttle stop screw (otherwise known as the idle adjustment screw, not the idle mixture screw).

[Norm B]

Post #3 mentioned it.
I haven't had chance to mess w/ it yet. The fast idle screw adjustment seems to make a lot of sense. I'll check the choke as well to make sure it's all the way open.
Missed that part of the post.
Did you change something, clean the engine or work around the carb area? Did someone lean on the air cleaner? Does the stuck high idle persist with the air cleaner removed?

Norm

[egchewy79]

to be transparent, this is a new carb on a 1979 f150 4x4 truck project I'm working on...not my cobra which has EFI. New Holley street warrior 600cfm going on a 335 series Ford 400ci engine.
I've had lots of pokers in the fire w/ this project and have recently gotten a new engine in, broken the cam in, and had to get the transmission rebuilt, so very little drive/run time.
One of the problems has been a slightly high idle, even when warm. Sometimes if I blip the throttle, the idle will settle down around 8-900 RPM, but otherwise will sit around 1300-1400.
I've let out the idle screw all the way and the throttle lever doesn't seem to go any more forward, so I'm trying to explore the other adjustments that could be made. Ill check back in once I get more time to work on the truck.

[Jeff Kleiner]

When I first read this I said, "Wait, Eugene has a Sniper????" Then I remembered the pickup! Sounds like you might be hanging on the choke's fast idle cam. Try assuring that is not happening by backing off this screw:

https://thefactoryfiveforum.com/attachment.php?attachmentid=211114&d=1741105746

If that solves the high idle issue get back to us ands we'll walk you through getting the choke and proper fast idle set up.

Cheers,
Jeff

211114

[Avalanche325]

One other possibility is the distributor advance being sticky. That can seem like your butterflies aren't closing all the way.

[egchewy79]

Finally had a chance to work in the truck and got the idle lowered by adjusting the fast idle screw. Thanks everyone

[FarmerDave1321]

Convenient time to revive this thread. I swapped out the 600 for a 750 double pumper today and HOLY CRAP! Needs some tuning (a bit rich), but it's such a remarkable difference in responsiveness and power.

[Norm B]

Finally had a chance to work in the truck and got the idle lowered by adjusting the fast idle screw. Thanks everyone

If the fast idle screw adjustment changes the idle when the engine is warm, you need to adjust the electric choke. The high idle cam should move enough that it is completely out of the way of the high idle screw on a warm engine.

Norm