I have a '69 427/ 390 horse motor that is stock with the exception of what I think is too much cam.

Anyway, when you attempt to set the idle it will easily idle at about 600. You can blip the throttle and it will return to 600 or so. If you try to raise the idle to 750-800, as you turn the screw very little happens until about a half turn of the screw, and then the idle raises to about 1300. With a lot of tweaking, I've finally gotten it to idle about 750-850 rpms.

When you tap the gas, and let it return to idle, it will settle in at about 1200, and in ten seconds or so slowly drop to 800. I don't think the linkage is binding, as the idle screw will be sitting on the linkage at any of these rpms.

This is driving me crazy. I've lived with it for too long, and hope that maybe someone can help. It's happened with two carbs that I've run on this motor. The first was a Holley reman, and the second was built by a local carb guy.

Has anyone else had this problem?

Thanks in advance,
Jim

Vacuum leak or the sec aren't closing completely.

I'd check for slop in the primary throttle plate shaft to baseplate. Common problem with a well used Qjet.

I wouldn't get too sweaty about a loose throttle shaft unless the throttle return springs are pulling up on the linkage, lifting it to the top of the bore and letting air leak in under it.

Your problem sounds like a vacuum leak or very lean idle/off idle condition. The sudden gain in adle RPM might have to do with the vacuum advance being activated as the throttle is opened, causing a big increase in the timing and idle speed. Is it the right carb for the 427? If it's off a small block it may not be able to supply enough idle fuel even with the mixture screws backed way out.

There's a couple ways to fix the lean circuit problem. both involve a teardown and careful drilling with special bits.

If you're unable to get it straightened out you can contact me via PM, I will fix it for you.

Incidentally, a 390/427 Vette was one of the best cars I ever drove. It was stock and had no bad habits, ran like crazy when opened up.

Thanks for all the input. I think you guys are onto something with the vacuum leak. I found a troubleshooting guide on the web, and they talked about how the aftermarket carb gaskets are cut so small that often you can get a leak under the carb. This makes sense, as it's done the same thing with 2 different q-jets.

The other idea I found was that the power piston spring may be too strong for the vacuum available at idle. The cam's pretty stout, so this may also be an issue.

The carb that's on it now is supposed to be a "hi-po" modified 800 cfm. The carb guy recommended it when I discussed the mods I'd made to the previous 750, which were drilled idle ports and drilled primary throttle blades, which were done in an attempt to get some idle mixture adjustability. These changes helped the 750, but it was never really right.

Thanks again,

Jim

Jim,

forget about the power valve spring baloney. the PV adds fuel to the main system, not the idle system. The idle system already has way more fuel available to it than it can use, the PV jiggling is irrelevant. This is an old magazine deal that's embedded in the hot rod culture so deep it will probably never get out. leave the spring completely out or put in a ball point spring pen that will hold the PV up under any circumstances, it will idle the same.

check for a vacuum leak at passenger side of the carb at the rear. there are gaskets that will cause a leak there especially if there's vacuum ported into the cavity in the bottom of the baseplate there. I've fixed at least a hundred of these over the years, very common when mixing carbs, manifolds and gaskets from different years and models. What you probably need is the idle feed resriction drilled out. It's takes a special drill bit and a steady hand. Also the idle channel restriction and the discharge port may need resized.

what year is the 800 you've got? is it old with a divorced choke or the newer style with integral choke and 13 screws holding the lid on?

Tom have you ever removed the idle feed restriction tubes? You don't need the long drill bits then.

Jim,you stated you thoght you had TOO MUCH CAM,what are your cam specs? Dur @ .05/lift/lobe separation angle to give an idea of what your setup is.

Without know the cam specs you said the q-jet was already modified for perf so it is hopefully set up to handle less intake vacuum @ idle.

As mentioned above its not your power valve,that would cause a rich idle /wasted fuel/possibly fouled plugs but not a fluctuating idle.

You also tried 2 carbs with the same results at ilde which is because the carb is does not look to be the problem from what you say is going on.

It sounds like from what you explain is happening that you have mult issues with one or all of the following going on at the same time which i will list below:

* Perf cam needs a lot more intial timing than the stock cams do and your running too retarded so bump your inital timing up to a minimum 15-16 deg btdc and some need 16-18 deg btdc,the hotter the cam timing is the more intial advance you need. The stock factory timing specs are no good for anything but the a factory stock cam and thats it.

* your mechanical advance may be setup to come in very early at low rpm and or the springs for the mech adv are worn/weak causing unstability problems at idle.

* Lastly,are you running a vacuum advance thats hooked to a full on vacuum source thats activating the advance at idle which can also cause unstable idle at the lower rpms especially if the mech advance is comming in at the same time @ idle. You should be able to find a ported vacuum source on your q-jet thats will not pull vaccum on the advance untill your out of the idle circut or just off idle.

I have very often found this to be the case when a motor is acting the way you explained it in your post so check all these things out as you usually have to dial these things in for fine tuning (esp for idle)when running a perf cam.

You can have a situation where the mech and vac advance are trying to come in at or just above idle speed which acan cause the idle to fluctuate a lot not to mention if your timing is also too retarded due to the perf cam with more cam timing/overlap just making things worse at idle.

If your not familier with this kind of tuning/dialing in just find a reputable performance ignition/distributor rebulider/dyno technitian in the area that will know just how dial in your intial timing for a perf cam along with the mech & vac advance too which should remidy your idle/low speed issues.

Let us know how you make out.

Scott

Tom and Scott,

Tom, the carb is a later style with integrated choke. I haven't run the numbers to determine the original application of the carb yet. I'm not that hung up on numbers, but eventually I'd like to find a more correct carb and have someone who really knows what they're doing build it for me.

Scott, thanks very much for the info in your post. Unfortuately, I don't have any idea of what the cam specs are, as the car was purchased this way. From my contact with past owners the motor was built in the mid-late 80's but I really don't have any details. Currently, it's got about 12 degrees of initial and 36 degrees of total timing. It's never had any issues with detonation, so it wouldn't scare me a bit to run it up and see what happens. Also, just FYI...the total timing was all in by about 1700-1800 rpm, but unfortunately I didn't note when it all came in, as I was by myself just working with a dial back light.

I've owned this car for several years, and never driven it much. I don't know what's happened lately, but it seems my fear of getting it hurt has dropped off (knock on wood!) and I just want to drive it. Hopefully I can get these bugs worked out and make it fun.

Thanks again to everyone for your suggestions!

Jim

Jim, by what you've described on your timing the springs must be real soft. My guess is that the mech. adv. is coming on @ idle or just above & slowly drops back. If no vac. leaks are found just put one stronger spring on & try.

Jim (77 cruiser)

I have used that method. it is do-able, but I work on a lot of more-or-less irreplaceable carbs and am very careful about doing things that are difficult to undo. I've had some that I tried like that refuse to come out, ended up having to drill them out and replace with parts from a spare carb. Deal is, I live in a large metro area and have easy access to the jobber length drills, so why bother? Of course, on the downside, I had a .038 drill break off down in the jet before, but only once.

Q-jet info---

http://www.442.com/oldsfaq/ofcrb.htm

http://www.florida4x4.com/tech/quadrajet/qjtshoot.pdf

http://www.florida4x4.com/tech/quadrajet/

http://www.tocmp.com/manuals/Carbs/...973_4M_0001.htm

I hope something here helps ya!


Pat