MaxxPayne

Having Fast Idle speed issues.

Ok so on a cold start car will idles fast to high like 2500 then climbs to around 3000RPM and takes a very very long time to kick down to 1600 rpm where it should be. When its warmed up it will Idle fine around 600rpm It's a automatic. I tried to adjust the fast idle screw and it does nothing. So I tried using carb cleaner to make sure anything was not sticking and even tryed to adjust the choke. Still can't figure this out. plugs and wires and dizzy is all new. not that it should matter.

What I have been doing to get by is I lower the curb idle on a cold start so its not so high while warming up but then after it kicks down I have to raise the curb idle back up because when its warm and kicks down the idle is below 500 and will stall. pain in my *** to mess with curb idle every time i want to drive this thing I can't figure this out

so basically it idles fine when its warmed up but during a cold start it idles way higher than it should be. Fast Idle screw has no effect what so ever. what gives.


79' Corvette.
Rochester Quadra-Jet 750 CFM Hot Air Choke

gcusmano74

To set the fast idle on a Qjet:
Warm up the engine and turn it off.
Remove the air cleaner.
Open the throttle a bit with one hand and reach around to the right side of the carb and set the fast idle cam to the high step.
Release the throttle, and let it close on the fast idle screw.
Start the engine without touching the throttle.
Adjust the fast idle as required. Find the spec for your car. Probably 1300 to 1600 warm. This will give a normal idle speed when cold.

Shark Racer

You're positive you have no vacuum leaks? I'd start by checking for vacuum leaks. If the fast idle screw is not responsive, it's likely something else, unless you've got a major bind somewhere.

Are your secondaries completely shut?

MaxxPayne

@gcusmano74
From what I understand I always thought that to adjust the fast idle screw, the engine has to be cold, b/c after its warmed up and it kicks down the idle operates off curb idle speed. so if its warm and I adjust the fast idle screw I will have no way to tell the difference until it's cold again. and that would take some time to cool down. to see if it changed after by doing it that way.
Because if its warm it fires up with no issues this only happens on cold start when i first start the car after it sits overnight to cool down each day. Upon first engine start of a new day the Fast Idle operates around 2000RPM's and raises a bit above that. diagram under hood says fast idle should be set at 1600. now i can bring it down on a cold start if I lower the curb idle screw behind the throttle on the driver side, but if i do after it warms up and kicks down I have to put it back where it was, and basically when turn the fast idle screw out/in on the passenger side under the choke adjustment the idle does not change at all.


EDIT: this is the exact same carb I have, and what the fast idle looks like on it btw.
Now when i turn this screw the idle does not change and I can't figure this out why I'm having the above issue. :/


@Shark Racer
Well i'm not positive there is no vacuum leaks, but I'm pretty sure all my vacuum lines on the carb are routed properly and there is no leak, and even if there was a leak somewhere I wouldn't really know how to diagnose or properly re do all the vacuum line routing etc. But I really don't think this is a vacuum issue because when the engine is warmed up the idle runs fine I can turn the engine off and it will fire back up later on through out the day as if there is no problem what so ever. its just that on a cold start when I first start the car each day that I notice it will Idle well over 2000 RPM/s and I know it shouldn't be that high. 1600 RPM/s is what the fast idle should run at. now if i let it sit for about 10-20mins (which seems unusually long for a kick down to occur to me) it will eventually kick down and run normal once its warmed up. the secondaries are completely closed. as well as the primaries until the choke opens them as the engine warms.

lars

The cam and adjuster on the fast idle system is most often seized up and stuck. This results in being able to turn the fast idle screw, but nothing will happen. You have to remove all of the fast idle linkage/adjuster components off the primary throttle shaft on the passenger side, clean it up, get the rust off it, lube it, and re-assemble it.

Lars

MaxxPayne

That's what I was sort of thinking because when I do like half turn or two full turns out to bring it down I don't notice any difference at all in idle. still stays above 2000. So I took a rag and used carb cleaner to spray the linkage down very generously. and its much looser now but still Idle's the same. I don't see really any rust. Should I try to back the screw out all the way? I really don't want to rip the carb apart or off the intake mans to try n fix this. If i push down on the linkage as said above in the youtube video should it kick down the idle instantly maybe it needs sprayed with more cleaner better to get it lubed more. when i first sprayed I only did the linkage should i spray where the fast idle screw is around there as well?

Shark Racer

Lars - do you have a recommendation for lube for the fast idle cam?

lars

Spraying cleaner on the cam parts won't help the problem - you have to physically break them loose.

If you look carefully at the fast idle linkage, you will see that the fast idle screw pushes on another lever which is linked and pivots with the lever that the screw is installed in. Back the screw way out, and see if the gap remains between the two parts: If it does, take a pair of pliers and squeeze the two parts together to close the gap. That will change your fast idle unless the fast idle cam itself is defective or hung up. Once you've squeezed the gap together, the fast idle screw can be screwed back in again to re-set the idle speed.

Lars

MaxxPayne

Yeah I was examining on my old q-jet laying around. the one that is on there now is a new re-man one only a few years old I really don't see how it would just stop working it only maybe sat in a garage for a year. but I was looking at the basics how it works. I'm kinda thinking the screw is in to far and I just don't notice a rpm change because I didn't back it out enough maybe. Also I wanted to ask you if u think the chock would have any effect of taking so long for it to kick down as I have it set to mean lean completely closed chock. would this also have a similar effect on taking too long for it to kick down / high fast idle. I'm gonna try and back the fast idle screw out all the way and raise the chock to more rich. I'm suspecting maybe the thermostat coil is old and worn and might need replaced as well.

off topic question what the vacuum advance from dizzy to carb do? I do not have a vacuum hose connecting the two both are sealed/capped off. just wondering

Shark Racer

Lars has a great paper for baselining your choke.

Vacuum advance is for adjusting the advance based on engine load. When you're cruising down the road, you have minimal load and the engine should be running relatively lean. Both of these factors allow you to run more timing advance. As load increases, vacuum to the advance pot drops and thus so does the contribution of vacuum advance. At WOT there will be no vacuum signal at most of the vacuum ports.

MaxxPayne

yeah I've heard Lars is the man around here, does he have a site with his papers or stuff or just somewhere on here in these forums.

I read somewhere to do the #2 pencil technique or if u cant find a pencil a drill bit works also. where I adjust the choke with a number 2 pencil down the carb and close it on the width of the pencil with just enuff space so its not completely closed up so you dont get black smoke or something I'm just lurking around trying to get more research before I actually play with it. as for the vacuum advance good info to know I was just a bit curious because I have the option but i dont have a line hooked up instead the ports are rubber capped. Kind of wondering whats it functioned and if it was strongly recommended.

Shark Racer

Email him for a fresh copy: v8fastcars at msn dot com