Idle hang-up

[William B.]

Gentlemen, Just completed a six year restoration effort. Looks great and runs like it wants to fly. She is a 327ci/300hp/Auto. Engine was totally rebuild as well as the Holley carb. Here's my issue. When started cold she idles about 600 and returns to that after a lite rev-up. After being driven and the engine is at operating temp and while stopping at a traffic signal or stop sign, she insist at idleling at 1200. A slight tap of the accelerator brings the idle back to 600. The linkage is free and not binding. There are no obstructions to the linkage to cause a hang-up. The accelerator is not pressing against the linkage. I've even added a second throttle return spring, and this doesn't help.

Where do I go from here? Any suggestions or SWAGs will greatly help.

Thanks,

Ken #42259

[Edward J.]

Re: Idle hang-up

Bill, have you taken a peak at the choke fast idle cam to see if its hanging up, or the butterfly to make sure that after a few minutes of running it is wide open?

I have seen when a carb. is rebuilt and the throttleshaft bushings are replaced this tends to make things a little tight ,causing a binding creating a high idle. But generally a added spring helps this.The other thing that adds to the binding shaft is, when the engine is running the vaccum created from the engine is pulling on the throttle blades to hinder and make the problem worst.

[John H.]

Re: Idle hang-up

Ken -

I'd check out the remote choke thermostat rod and the fast idle cam. At cold start, it should idle about 1500-1600 rpm in neutral or park if the choke is working properly until it warms up, and after warming, the choke blade should be fully open and the fast idle cam should drop down completely out of the way. Sounds like something is awry and the fast idle cam is engaged by a couple of steps when it's warmed up, and drops out when you tap the accelerator.

Photos below show the fast idle cam position "off" and "on" (choke rod from lever to blade removed for clarity).

[Joseph K.]

Re: Idle hang-up

I agree. I had the same problem

[William B.]

Re: Idle hang-up

Thanks Guys for the timely response. I'm not a carb expert, but I'll get this checked out.

[Timothy B.]

Re: Idle hang-up

The problem could also be the throttle blades not positioned as well as they should be in the base plate.

If the cold start has the engine RPM at 600 the choke is not working properly.

[Tony S.]

Re: Idle hang-up

You might also considering a slightly heavier spring. I've had the same problem with my '65 L79 car. I have a "judging" spring from Long Island that isn't quite strong enough to pull back on the carb linkage. With the judging spring, I have to punch the accelerator pedal to bring the idle back to 800 after I've driven it for a while.

I also have a slightly stronger spring that I use for weekend drives. It's strong enough to pull back the linkage, so no need to "punch the pedal".

Tony

[Michael G.]

Re: Idle hang-up

Anthony,

William indicates in his initial post " I've even added a second throttle return spring, and this doesn't help". Wouldn't think adding a stronger spring would be the long term fix. Most likely will be a simple adjustment of some sort, i.e., linkage, choke, flaps...etc.

[William F.]

Re: Idle hang-up

Had my 3810 Holley rebuilt by Eric Jackson-Vintage Muscle Car Parts ,who did an excellent job but still have a slight case of the problem described, Can't remember the details, but he says it's something about the throttle shafts that there's no proper tool/drill(?) currently for proper fix.-Not unusual in a rebuilt carb, even if done properly. Might want to give Eric a call for further explanation.

[Jephrey S.]

Re: Idle hang-up

If the hot idle is steady at 1200 rpm, I doubt that it is the return spring. The rpm's should continue to climb if you are actually at part throttle. If the transmission is in park or neutral and holds at 1200 rpm then I would look at the carb adjustments suggested.

Adding a second spring is also not a good answer if you do have a throttle return problem. If you have a mechanical bind that cannot be removed for whatever reason, then you should go with a shorter spring with the same spring constant or the same length spring with a higher spring constant. The double springs will end up with their coils meshed together. At half throttle this will give you the double return force that you expect. But at only 5 or 10% throttle, the meshed springs will result in an effectively longer at rest length and you can end up with less (or no) return force where you actually need it. I had to safety wire the center coils of my throttle return spring together when I could not find a replacement that would work. This shortened the effective spring length and gave me enough pull just off idle where I needed it.