First let me start by saying, I love this site! Tons of info and tribal knowledge available. I'm somewhat of a gearhead and I know my way around engines, etc. But I'll freely admit, I'm no expert.

Here's my issue and recent confusion. I'm trying to tune out a slight miss on idle. It idles rough enough to have a slight shake in the hood when it's up and it smell like it's running rich.
72 Cutlass, Automatic, 350 Quadra-jet, AC etc. 130K on the clock. Basically bone stock with dual exhaust. Just had the carb rebuilt, which was a huge improvement.

Reading up here on tuning and dialing in the timing and carb settings, I'm supposed to have ~20* of vacuum at idle?
With all the vacuum ports pulled and plugged, I've set the timing at 12 degrees, which seems right based on the performance and drivability. When I connect a vacuum gauge to the manifold port behind the carb, the most I get is about 12 at idle. Cracking off idle it jumps to 15 plus. I've replaced the plugs and wires with NGK's set to 35 and high end Belden wires. The points, cap and rotor are clean.

Since the car is 50 years old, I expect some dry rot, so I've replaced every vacuum hose I can find. I've stocked up on spray cans of carb cleaner and I'm chasing all the leaks.
Am I misunderstanding something here or am I in the ballpark of where it should be?
Thanks in advance for any thoughts and advice.

 

With no modified cam, bone stock you should be able to achieve between 17” - 19” Hg. I have same engine in my 1971 CS convertible - I achieve 17.5” Hg. You have the TCS solenoid/valve still installed? Sure you’re hooked up to correct vacuum line/port on carb? I use a tee inserted into a vacuum line to measure.

 

I noticed you have all vacuum lines plugged and pulled. The normal operating state (all vacuum lines connected) is how I measure vacuum since that’s how the engine operates. Did you actually pull/plug the brake booster vacuum line? You did say you pulledlugged all vacuum lines. I never measure vacuum in the manner you did mine are all connected and I use an in-line tee.

 

Norm,
Thanks for the quick reply. I'll check on all the connections again but I'm sure everything is where it should be. I'll try your "tee" method and see if that makes a difference. It still has the Distributor Vacuum Control Switch installed so I'll double check those connections are correct. Also with everything connected, I want to hit it with carb cleaner and see if all is tight.
Thanks,
Mark

 

The vacuum signal should not matter if lines are plugged or connected, other than the distributor vacuum advance (assuming there are no vacuum leaks).

It's been a few years since my engine had a stock cam (like 30 or more) so I don't remember if I measured with the vacuum advance connected or not - with the stock cam and finely tuned engine, it pulled 21" vacuum at idle. My current engine with its 217/221 cam pulls 15-16" vacuum with the vacuum advance disconnected, and 17+ with it connected.

Have you adjusted the idle air screws to obtain the highest vacuum readings? Those screws need to be adjusted after a carb rebuild since everything was disassembled for cleaning.

 

Agree w/ Kenneth. I wasn’t suggesting you’re measuring incorrectly, I prefer to measure with all vacuum lines connected.

 

I didn't measure the vacuum on my engine before just recently rebuilding the carburetor but now it is at 19 inches at idle with distributor advance un-hooked and the gauge in it's place. One of the big reasons I rebuilt the carburetor was because it was way, way, way rich and smelled very gassy. And ran poorly. I found coke build up in the metal tube at the back of the carburetor that goes to the pcv valve, the one on the baseplate, the idle mixture orifices were also very nasty with carbon build up. The choke plate would not open fully because of carbon build up. I describe all this to suggest that if you carb is in a similar condition (based upon your description of the smell) this may prevent you from getting a good idle mixture setting. This is just my experience and it might not be what you are having trouble with but maybe.....

 

Certainly agree w/. Dan. Let’s see what your carb cleaner around the carburetor reveals. Generally the base gasket loses integrity over time but if you find the carb is the issue, installing a rebuild kit is super easy since you’re removing carb anyway.

 

Just thought I'd post an update to the vacuum issue on my 350.
Based on all I've read on the site, I decided to bypass the Distributor Vacuum Control Switch. I plugged the port on the carb, ran the hose directly to the manifold vacuum using the "T". So now the tranny vacuum, and distributor are direct to the front 1/4 inch vacuum port on the manifold. I wanted it to still look stock so I cleaned up the switch, ran a hose from the top of the switch to one of the ports with about a 6 inch line and plugged the connector back in. The manifold vacuum is steady at about 15 and the advance on the carb is working as it should. Not sure what's going on there, but I'm moving in the right direction.

I still think I have a leak somewhere, since when I spray carb cleaner around the back of the carb it effects the idle. There's a small vertical plate at the rear of the carb. According to the manual (something to do with idle) with a spring loaded pin sticking out? When I hit it with carb cleaner it changes the idle. Not sure if it's meant to or not.
Still investigating but moving in the right direction. It idles much smoother and it runs way better

 

Did the carb rebuilder mule test the carb before it was returned to you? I ask because the throttle plate can be problematic on these aging carbs. The rebuilder (well-known) of my QJ put it back together, tested it on an engine and it idled not to his expectations. He had to take it back apart and bush the throttle plate because initally he thought things looked good, but the engine test proved otherwise. You may want to approach your rebuilder for more information about exactly what was done to your carb.

 

The thing on the back of the carb is the hot idle compensator. A bimetallic strip in the mechanism opens an additional air inlet when the underhood air is hot and less dense. It's possible that the gasket on it has deteriorated.

 

^^^^ This^^^^
Certainly the rebuilder should have replaced this gasket. It is a small cork-like composite gasket in most kits. Again, talk with your carb guy to know what he did; not all rebuilders are created equally.....

 

Good info, thanks!
My next step was talk with the rebuilder. I took the carb to him to rebuild so he never saw it on the car. I assume he changed all the gaskets, etc. Might be time to stop by with the whole car.
I did replace the manifold gasket with a brand new one but it could be leaking.
Thanks Joe, for the blurb on the Hot Idle Compensator. I'll run that test next and see is it's malfunctioning.

 

Not that I think your rebuilder needed to see your carburetor on your car, but in my opinion, he sure needed to test it before he returned it to you, which he may have done, or not. If shortcuts were taken and if gaskets were not replaced, like Joe P. indicated, any overlooked gaskets could be dried and dusty at this point. Your test indicates a leak at the rear of the carb and it very well could be a fault at the compensator, or other anomaly.

Please keep your thread alive as you step through this issue as your experience will help someone else down the road.

 

My “Quadrajet Power” sits on a shelf, where it belongs.