1966 Chev Impala - rough idle

My 1966 Chev Impala has a 327 CI engine. The engine was rebuilt to stock specs and has about 20,000 miles on it. It has always idled rough when cold and hot, before and after the rebuild.
Sometimes, it will idle okay when warm but most of the time, it is rough and the  the degree of roughness varies.

Here are some things I have done to attempt a fix:
1. About 3 years ago, the compression tested okay. I put only 2000 miles a year on the car so I assume this is still okay.
2. In October 2008, I installed a new Edelbrock 600 Carb.
3. In October 2008, a new distributer and coil was installed. This converted the ignition to electronic.
4. We also replaced the fuel pump, spark plugs, and plug wires at that time.
5. Insulation has been wrapped around the fuel line from the fuel pump to the carb.

All fine tuning (timing, carb adjustments, etc) were made.

I am looking for some economical advice on what to try next.

My friend suggested these possible remedies:
a. Install a outside air intake to the air cleaner so that the carb uses "cooler" outside air instead of the engine compartment heat.
b. Could there be a vacuum leak around the intake manifold ?

All suggestions are welcome, thanks !!

MN  

What cam was put in it hydraulic or soild lifter style.
Does it smoke any.  
Horsepower sells Engines and torque wins races.

1. I am not sure about the Cam. The engine was rebuilt to stock specs....is that typically hydraulic for 1966 ?

2. It does burn a little oil when cold.  

do you have a vacuum gauge????   that looks like a harbor freight ITEM 93547  ... almost every parts store has something like it...

hook it up to manifold vacuum...     see how much the needle moves...  there are charts in most manuals to describe what each problem looks like with a vacuum gauge...

the dashboard mounted gauges are with a long tube that restricts the movement of the gauge... so you get a nice average instead of seeing what pulses are happening...

do you have the # 5 and # 7 spark plug wires separated?????

is there any chance that the cap is cracked...   or there is carbon tracking inside the cap...

do all the spark plug wires look OK... no broken boots... no burned through spots...   opening the hood at night and running the motor you will sometimes be able to see arcing from the wires...  a bad thing...

i take it that you have changed the spark plugs... and have the proper ones in it...    there are gasket seat spark plugs... delco.. R44S...   13/16 hex...   and  tapered seat spark plugs...  R44TS... depending on the year of the heads... either one could fit...   you really need to examine the machined area where the spark plug threads into...  the gasket seat heads will have a large area that is flat... with almost no taper at the entrance to the spark plug threads...

the tapered seat heads.. with have a wide 45 degree taper above where the threads start...

how about vacuum leaks from the intake manifold...

which intake manifold do you have???????   the 64 through early 70's has    exhaust gases flowing around under the carb...  and uses a stainless steel shim under the gasket to protect the carb...  if these get misinstalled..  and they *** it will idle really bad...   the steel shim goes right on the intake.. not above the gasket...

if the shim goes above the gasket... it lets exhaust gasses up through the forward carb mounting studs...  

Hi and thanks so much for your time and info.

I will check all the itmes you mention above.

As I recall when I replaced the carb last year, the manifold has many "passages" so I think that the exhaust gases do flow around under the carb as you stated.

My friend and I suspect a vacuum leak in the Intake Mainfold area. We need to check that.
The rough idle is very inconsistent. By that I mean that the engine idles rough most of the time. However, the "roughness severity" varies hour by hour or event by event.

Thanks again !  

I am experiencing he same problem with my 66 impala and have a chevy 350.  So just to clarify the steel shim goes directly on the manifold with the gasket on top?  Do you use any gasket sealer between the steel shim and the manifold?  I have a RPM intake manifold (high rise).  

The intake is probobly leaking, most likely on the front or rear valley seals. Check your vacuum,if vacuum rapidly fluctuates you probobly have a vac leak, the vacuum should be steady. If it was a stock rebuild you should see about 21-22 inches of vac at idle.  

broncoman94706     do you have a factory intake or an after market high rise performance manifold like you sort of describe?????

only the factory intakes used exposed heat directly under the carbs...  with a stainless steel shim gasket to contain the exhaust flow...

the shim goes straight on the intake.. the matching gasket goes on top of it...

broncoman.. i don't thing that you have this leaking problem ... if you have an aftermarket manifold..

you still might have one that the gaskets have slipped out of place when they were installed..  or the intake was installed off center...  

i have taken to using black superweather strip adhesive to glue the end seals to the top of the block.. then  glue around the water passage openings on the head side of the intake gaskets..  and use 2 bolts with nuts part way up the threads to hold the side gaskets in location..   but only 10 minutes prior to the intake install so the sealer is soft enough to still move slightly when the intake is installed...     i also start the intake bolts with a cross pattern staring in the center..  but never doing one side completely... then the other...    back and forth.. back and forth.. X patterns...   be sure to stick the distributer in  after you get the bolts in.. but before you tighten the first one...  you do want the distributer to be centered in the intake manifold hole...