I have a 75, L82, Auto, A/C with 76,000 miles on it. Car runs pretty well but I can't get a smooth idle. Car has new plugs, new wires and new distributor (the entire gizmo). Someone suggested to me that I might have a vacuum leak causing the rough idle. I doubt that it's leaking at the carb (edelbrock carb bolted the edelbrock intake manifold). But I have vacuum hoses going everywhere and since they are all the origional hoses that's the likely place to check if I have a leak at all. Any suggestions oh how to approach this and where to check first. (engine is stock except for the two edelbrock parts mentioned above)

One other piece of info. I've checked the engine vacuum with two different vacuum gauges from various locations and the max reading I get at idle is 9 in./Hg. That seems way too low doesn't it, and someone told me that an engine wouldn't run with vacuum that low. Any ideas here. ---- thanks

 

9" is very low for a stock L-82 (My 1973 L-48 with Edelbrock manifold pulls about 19-20")
Get a couple of pairs of forceps and start pinching lines off at the intake manifold (just behind the carb) and see if the idle smoothes out.
If not, then start from the connections off the intake pinching them off one by one in a linear mode until you hear a difference in the idle..
It can take a long time but you will find the problem sooner or later.
It took me about an hour or so to find my vacuum leak using this process.

Also, I would not rule out the carb or the intake manifold gaskets.

You can use a propane torch which is NOT lit, crack the valve open (just enough so you can wave it by your nose and smell the propane) and run it around the carb and intake while the engine is idleing, If the idle raises you have found a source of vacuum.
Be sure to do this out doors...

Some people use starting fluid or even WD-40
I have used starting fluid but there is little control over the flow and the WD -40 makes a mess

If you are not comfortable with the above suggestions than take it to a shop
It has worked many times for me.

If you need help you can PM me
I'm not far from Norwalk

 

Good suggestiions ---- I will try this and see what I find. It may be the big hose that goes to the power brakes. Also, my headlights work on vacuum, but as soon as I shut the engine off, the headlights won't move up or down which means there is no stored vacuum anywhere and it seems to me there should be some stored up vacuum. Leaks somewhere and I'll give your methods a try this weekend.

Thanks

 

The headlights are the prime suspect for vacuum leaks. I suggest you just cap off the port behind the carb...this is the one that goes to the headlights and see if it makes a difference.

There are alot of vacuum lines and connections that run off of this port. By capping it off you can either find that the leak is somewhere along this path or not. Its a guick check. Just find a rubber cap for each line coming out, plug them all and then put one back at a time.

My $0.02 worth anyways.

LannyL81

 

vacuum leak is a distinct possibilty somewhere in the system but so are other possibilities for you to check such as a faulty EGR valve or PCV valve.

One thing that strikes me to look into is your distributor. You said you replaced the entire distributor. Are you running the correct vacuum advance can based on your motors vacuum level? also, is it connected to ported or full manifold vacuum?

 

 

The Vacuum advance can on the distributor is the one that came with the entire "Printronics" distributor set-up. It is connected to a ported vacuum connection ----- mainly because a full vacuum connection wasn't available. So there is no vacuum going to the vacuum advance at idle, so this couldn't be the problem. I think what I will do is disconnect ALL vacuum hoses and plug all ports, except the one going to the power brakes and the one going to the automatic transmission. Then see if that solves the problem and if I can get a higher vacuum reading on my vacuum gauge and try to trace down any leaks. There could of course be vacuum leaks in the transmission connecion (although the car seems to shift and down shift well), at the manifold, along the hose or at the transmission itself. Lots to check out and I hope it's just a simple leak and nothing more.

 

For some reason my 73 refuses to run correctly on manifold vacuum
It needs to be on a ported source

 

It should be on a ported source

 

it should be on what makes the car run best but TYPICALLY most cars will run better on full manifold vacuum - better throttle response, lower operating temp, better idle characteristics. Again, not always but usually.
The ONLY reason GM switched to to ported vacuum for the vacuum advance was for emissions reasons.

 

Your vacuum is low, follow previous postings, very good information, if you’re still having a problem check for a leak at the intake manifold and the heads using oil can filled with motor oil, if there is a leak at this you will hear it both with a suction sound and change in engine idle.

 

Not meaning to hijack the thread but...

Barry, on manifold vacuum the advance not functioning.
On ported, it is waiting for RPM to release the can causing advance... Is this correct or do I have this reversed?
If so, it makes no sense to me

 

could be that your idle mixture is just way out of adjustment. Could also be a timing issue, or combination of things.

1. Find and fix any vacuum leaks.
2. Verify Timing
3. Adjust idle mixture and speed to max vacuum at 850rpm.

If you do the above and still have low vacuum, time to start checking valve train and chambers.

 

Good question

happiedazs, do you have the EGR on the Edelbrock intake or did you purchase the non EGR?

 

Bob, on full manifold vacuum you get full amount of vaccum advance at idle. This is because the vacuum is coming from a port on the carb below the throttle plate and since it's below the throttle plate it sees full amount of manifold vacuum at idle. Once the throttle plate is cracked open (by pressing down on the accelerator therefore opening the throttle plate) the vacuum starts decreasing and therefore the amount of vacuum advance starts decreasing. The more the throttle plate is opened and the more load the motor is under the less vacuum the motor shows until at WOT there is virtually no vacuum in the system therefore no vacuum advance being supplied to the distributor.
On ported vacuum, it works the same way EXCEPT that the vacuum port on the carb is located above the throttle plate so that at idle it sees NO vacuum therefore at idle you get no vacuum advance to the distributor but as soon as the throttle plate is cracked open just slightly it will see a high level of vacuum to supply full vacuum advance. Once this happens it now acts the same as described above - the more the throttle plate is opened the less vacuum it sees so it supplies less vacuum advance until the motor reached WOT and/or under a heavy load where there will be virtually no vcuum and therefore no vacuum advance.
If your vacuum advance is working on ported vacuum but not working at all on full manifold vacuum than you may be connected to an incorrect port for full manifold vacuum.
Any other questions on this PM me or start another thread please so that this thread doesn't get hi-jacked any further.

 

thanks, ill start a new thread......shhhh
Sorry for the interuption

And now back to our regularly scheduled program

 

Check your power brake hose and connector if you have power brakes.

Bill

 

but do this all with the vac gauge attached as you time the car and adjust the carb . Thats how I got the highest vac reading I could, still only 13-14 but better than 9,
ESU

 

As far as checking the carb and manifold, isn't there something you can spray or pour near the gaskets or connections that will get sucked in and increase the idle if that is where the leak is? I vaguely remember something along these lines.

 

Carb cleaner, starting fluid, the theory is the idle will raise when u hit the leak cause its temporarily closed. The sprays make it easy to get to hard to reach places.
ESU