My car's at a (good) shop but even they're having trouble sorting it out, so I thought I'd solicit input here and see if anyone has any similar experience.

In short, it's a ZZ502 big block that ran "OK" before but that I just converted over to factory 427/390 intake and Q-jet for a more original appearance. Since doing so it will not "idle down" below 1000 without a ridiculous amount of timing (like 22 BTDC).

The engine has the original ZZ502 cam, which is 218/230 - so it's stouter than a stock cam, but not wild enough that I'd expect extensive carb modifications would be needed. I know the Q-Jet, at 750cfm, might be a little small for the application but the problems are at idle.

With normal timing (10 BTDC) it just dies unless you crank the idle open so much that the idle mixture screws no longer respond, probably because it's then running on the main circuit. When it's like this the idle screws no longer do anything (you can turn them in all the way) as you'd expect.

After running out of obvious things to try they drilled 1/8" holes in the primary blades, which helped, but it still wants a ton of timing. They then started limiting mechanical advance (because they've got 15 or more degrees of initial timing), but that seems like a band-aid to me.

There are no vacuum leaks that they can find, but I suppose there could be one under the intake. The ports on the ZZ502 heads are not exactly oval port (some call them "roval" ports) so they do not exactly match the intake, but from the way the gaskets fit during assembly it did not appear they would be an issue; they're very close to oval ports and the gaskets looked like they would cover everything.

If you've been down this path before with an engine that would not idle down correctly, and solved it, I'd love to hear what your case turned out to be.

 

No experience with your particular set up, but it sure sounds like a vacuum leak to me, either at the manifold, or possibly internal to the Q-Jet.

 

Easy vacuum leak test: Back out your idle speed screw. If it does not idle down and die you have a vacuum leak, cuz air must be grtting in somewhere. Simple, eh? Lars taught me that one.

 

Where is your vacuum advance connected to, manifold or ported?

I had a similar experience and changing the vacuum source signal made the difference for me. If you are on ported now try manifold or vise versa.

 

The vacuum advance is connected to the correct timed port.

My intake, which is 1969, has the exhaust crossover "trench" up front; the carb is a mid-70s that does not have the slot in the carb. They do -not- have the stainless steel shim gasket installed yet, but they said that the single gasket they have in there appears to be imprinted well and looks to be sealing.

It'll burn through eventually so I have to add the steel shim anyway, but could it cause a vacuum leak because the steel shim gasket isn't there somehow? I'd like that to be the cause, but can't think of a way it'd do it.

 

Try manifold vacum. My engine ran great with manifold vacuun with qjet but when I switched to holley it needed ported. Idk

 

Check the mating of the base gasket to the carb and also to the intake very carefully.
You do need the steel gasket or use pipe plugs to seal the 2 holes in the manifold crossover.
Your idle circuit needs to work properly. What year is the carb from?

 

I agree. I have been working all week to get my new 383 back to running well. I had it running like a top then went back to the garage to tweak some hoses, reinstall the power steering, and some odds and ends. I then started it to find that the engine would die as soon as the engine was warm enough to drop the choke. I went through every headache with timing and ignition before I found a vacuum leak off the intake to the transmission. Good luck hunting!

Mike

 

I know I need to plug the crossover or get the steel gasket or it'll burn through, I'm just wondering if it could cause a leak even -before- it burned through.

The carb is a "Jet Stage 2", so it bears little resemblance to its initial as-produced self, but I think the core is from a 75.

 

Any way to track down an in-the-valley leak? Haven't been able to locate one anywhere visible, but I'm wondering if you could plug the breather and PVC to help find one, or if blow-by would negate any such attempt anyway.

 

Could you nave a vacuum advance module (the can) that isn't compatible with your set up? I had to change mine to pull advance at a lower amount of vacuum. If I adjusted my idle below 1000 rpm, it would eventually idle down and die. It was because there wasn't enough vacuum to hold the advance. Didn't matter if I used ported or manifold vacuum. Going to a can that pulled advance at about 8 inches solved the problem. You can test this by using a hand vacuum pump connected to you vacuum advance. See if after you set the timing then you put vacuum to it, see if it'll hold an idle. Worked for me....

 

Unless you bought and installed the intake manifold gaskets that match your HEADS, the squarish, not the oval style, I can practically guarantee that's the source of your vacuum leaks. I've seen this a couple of times with the GM?Edelbrock 'roval' heads. The embossed areas of the oval gaskets have nothing to seal against the corners of the heads.

Easy test, with the engine running, spray the gasket areas above the intake ports with carb cleaner. If the engine rpm picks up, you've found your leak. Works for carb pads too.



Edelbrock p/n is 7203. Note the black embossed areas.