I have a 80 Vette that has a rough idle maybe a miss below 1800 rpm then it pulls great. The only vacuum hoses are the PCV and breaks. (it ran the same with all vacuum lines disconnected) I have replaced all the wires and the distributor with Hp. What am I missing?

Do you have the original Quadrajet carburetor?
Have you attempted to adjust the idle mixture screws?
What spark plugs are you using, manufacturer and part#? Should be AC-Delco R45TS
Do you have the spark plugs correctly gapped? Should be ,045.

I have a JET Quadrajet and another rebuilt Quadrajet, I am using the AC Delco plugs gaped to 45 I have tried different plus, wires, carbs and distributor with the same result.

Put a vacuum gauge on it at idle......does the needle bounce?

Jebby

The mass produced rebuilt carburetors have a very poor reputation. Contact Lars at v8fastcars@msn.com for a paper on the issues found in these carburetors.

What port do I use for the Vacuum gauge? Do I put it on manifold or carb vacuum? If it does bounce what does that mean?

Use a port that will not affect timing to the dizzy and preferably full manifold VAC.

Uneven needle indicates a sticky valve.(maybe)
But yours runs great above idle. So its not plugs, wires, plug-gaps but rather something that occurs only at idle. If the IGN system is going to mis it should be under load / high RPM.
What's the history on the last carb cleaning? You may luck-out with a whole can of Seafoam in the tank and a half a can down the carb at part throttle.

The jet carb is new and the other is a resent rebuild. It runs the same with either carb.

The heads sat NEW on the shelf for years. Put them on a couple of year ago.

It means you have a vacuum leak created either by carb base gasket, intake manifold gaskets or a bad intake valve.....
Any of these could clear up as the rpms rise....
Vacuum is highest at idle.....with the throttle blades shut. Piston events will happen faster than the vacuum can suck through as the rpms rise.....thus clearing up....
Put it to direct manifold vacuum.

Jebby

Normally I would say: what's the odds that BOTH carbs won't idle. But because of poor Q-jet history with some rebuilt carbs, I would say really good odds.

Just for peace of mind, follow number 5 plug wire and number 7 plug wire from the dizzy to the plug. Because of the SBC firing order, those two wires should never be near each other (crossfire). Separate using looms. I doubt that is the issue, but no harm in looking.

I have 15 vacuum and the needle doesnt bounce. I have removed all vacuum lines except power breaks and PCV. I sprayed carb cleaner around the base of the carb, intake and power break hose. I don't hear any drop in RPM I dont seem to have a vacuum leak. What do I try next? It seems to run fine when I first start the engine but after about 1 1/2 miles it starts to get noticeable and after 2 miles it gets worse.

Have you set timing to 36 degrees at 3000rpm with the vacuum advance unhooked?
Disregard the initial timing.....set to 36 degrees at 3000.
Post your results.

Jebby

Quote:
I sprayed carb cleaner around the base of the carb, intake and power break hose. I don't hear any drop in RPM

Idle RPM will go UP, not down, if the carb cleaner is sucked in through a vacuum leak because it adds fuel to the idle mix. Idle RPM goes down with a vacuum leak because it adds air to the idle mix.

Is the EGR system still installed? If so a defective EGR valve or dirty EGR valve pintle will cause a rough idle that I [think] gets worse as the engine warms.

Look for the "T" in the PCV hose that runs from the driver side valve cover to the carburetor. Block off the tee branch leading down and towards the driver side fender. It leads to the EVAP (fuel vapor recovery) system.

No The EGR is not hooked up... It's gone. The PCV hose, I have blocked off all other hoses. only PCV to the carb. There are no hoses except the Power brake and the PCV to the carb.

Ok I set the timing to 36 at 3000, It seems to have worked. It wasn't that far off. What was going on?

What is “not far off?”
10 degrees on the pointer isn’t much.....little timing, big gains....
I am glad it is running better and yet another testament to the importance of proper timing.

Jebby

Maybe 5 Degrees. How would that affect Idle?

Because engines like advance.....especially at idle.....5 degrees is not a lot....but maybe it did the trick....

Jebby

Ok I spoke to soon. It runs better but, It will still stumble "not as bad and it takes longer to start doing it" What's going on?