So, I just got a Chevy truck with a 350 in it. It is mostly stock with an HEI distributor and an old Quadrajunk. It ran like **** when I picked it up and I fixed a couple vacuum leaks initially. That helped a bit, but when I was trying to time it, the idle would not stay constant. I tried adjusting the idle mixture, and it was WAY to rich. After that, I'm lost. I have no idea how to smooth it out. Could there be a vacuum leak in the carb somewhere? Or is another internal carb issue? Any help would be great.
Take a can of starting fluid and start spraying around where there would be vacuum. If the rpm increases, there's a leak somewhere in the general area where you sprayed.
Might also lower the idle with the screw on the carb. Get it as low as you can. If it steadys out, check the springs and weights under the rotor. If that gets gummed up or the springs weaken it will let the mechanical advance come in and out at LOW speed.
Josh, glad your a HAMB Chapel member and believer, but please dont call the Rochester spread bore junk. It is actually a very misunderstood EXCELLENT carb. If the previous carb owner tightened the **** out of the air mixture screws, which could very well be your problem, it's not the carb's fault. A monkey with a screwdriver can ruin a $1500 Brazwell carb in seconds. Carb's, any brand carb's, are wrongly blamed 99% of the time for a driveability issue. Good luck brother, you have already been given some great advice, BDM.
I went through the same rich idle unstable idle with a friends Vette. Another friend said it sounded like "an internal vacuum leak inside the Qjet" I never heard of that term. He claims to have seen a few with warped castings. I had the carb apart for a kit and did not see anything like gaskets not sealing, but a rebuilt carb fixed it. I don't like to buy rebuilt carbs without seeing something really wrong, but the guy wanted it fixed quickly. it almost sounded like a moderate high perf cam at idle, ran rich, and could not keep it at a steady rpm. Idled like a Caddy after the swap. EDIT; another thing on that car was the carb vacuum noise with the air cleaner off at idle, really sounded strange.
As a further note to what dbradley said- Older HEIs are notorious for suffering internal arcing from the coil. This often arcs to the centrifugal advance pins, causing them to literally erode away. If you have red "dust" inside the cap, it's happening. Look at the pins and weights very carefully for wear. This will cause erratic timing, as he noted, because the centrifugal advance is constantly changing and prematurely occuring at idle. The cure is to replace the internal coil...and the distributor, if the pins are eroded. Personally, if it were mine, I would check the HEI as a first step (I've found this arcing problem many, many times). I'd further inspect the ignition system for proper operation. I'd then search for a vacuum leak as others have noted. Only after that would I touch the Quadrajet. Carburetors are generally the first part to be blamed, when the ignition system is often the true culprit. I personally like the Quadrajet very much and have had a lot of luck with them. One of these days I'm going to set up another tunnel ram with Quadrajets, just to shock the naysayers. They work astoundingly well on a tunnel ram when tuned correctly.
Hey, good deal. I am fond of the HEI and "back in the day", before all the aftermarket versions were available & Mallory/Accel distributors were relatively expensive, I would use them a lot for budget builds. The first thing I did, after inspection, was to use a tan Accel cap & rotor, with the appropriate Accel coil...or use the MSD kit to completely remove the coil from the cap altogether. A good, properly set up HEI with these mods will easily and reliably work past 7000 rpm with no problems. Nowadays it's more cost effective to just buy an aftermarket copy from MSD or whoever....as long as it's not a ProComp or cheap generic version.
With the engine warmed up and idling, open up the choke valve on the primary side if it isn't already and make sure there isn't any fuel dripping from the main nozzles. Open up the secondary air valve as well and check for the same thing. Could be a sunk, stuck or misadjusted float, or a needle and seat that aren't quite sealing properly. Check that all the screws are in place holding the airhorn to the bowl. Give them a little "snug" if they need it but don't overdo it. Check to see if the power piston moves freely and make sure someone didn't bend the tips of the metering rods. Get a vacuum gauge on it to manifold vacuum and see what that tells you.
Note about spraying stuff to find leaks: never use brake clean!! You will die. As to the idle - check the valves, too tight can cause this. Cosmo
Along with advice above, on an old quadrajet, I would also look at the float, the old black plastic ones got heavy, (gas-saturated) causing running rich. If you have a choice go with new br***. There is a weight spec for them, but it's easy to compare with new float at parts store.
Well, upon further diagnosis, I don't think the carb is to blame. The mixture smells really good now, not rich at all any more. I am going to look into the HEI issue that Homespun91 mentioned and the condidtion of the weights and springs. It's in a truck that has gotten muddy and wet alot of it's life, so the arcing seems like a very logical possibility. I just hate crawling across the engine bay, stupid Chevy. (it's a joke, don't anybody take it personally Either way, THANK YOU ALL SO MUCH!!! The HAMB rocks, this is exactly why I am an Alliance member. Viva La HAMB!
Something that has not been mentioned yet is the need to check for play in the throttle shaft and bushes. you can check this simply by taking ahold of the arm on the end of the shaft and attempt to move it up and down. If it moves then it is worn and will cause an air leave and you will not get it to idle properly. If worn then best to fit a reconditioned carb.
I don't know the Quadrajet, but I had a Carter that was doing the same thing. It turned out that the accelerator diagram was split and leaked directly into the throat of the carb. Probably a different design, but maybe it isn't .
Look, if you really don't wanna crawl ALLL the way back to adjust the distributor, put a Cadillac in there!! The distributor is up front, and the torque is up there, too!! And they fit!! Ask me how I know Cosmo
The Jalopy Journal
