Rough idle- low vacuum

[Sagesurfing]

So I got a 1972 K5. It always had a bit of a rough idle. It bugged me and so eventually I swapped out the engine for a new crate 350 from Blueprint. The only parts I didn’t swap were the intake manifold, the carburetor, and the distributor. But the new engine still had a sloppy idle. The kind that is slightly inconsistent, that throws the timing light off every so many bleeps. Anyway… since then I’ve swapped the carburetor (for another same Edelbrock 1406,) swapped out the distributor, taken out and reinstalled the intake manifold, studied the carb in depth and learned everything about tuning it, and hunted down every possible place I can think of for vacuum leaks (around the intake manifold, base of carb, surface of intake, valve covers, disconnected any and all hoses and lines including break and transmission,) and yet… rough idle and vacuum at 15 jumping around a bit as it sputters. When I accelerate it sounds and runs great. But to keep it from dieseling I have to run out the air mixture screws 2.5 turns. I set my idle rpm at 800. What else could it be?

[geezer#99]

Lack of enough initial timing!
What’s your timing set at?

Set your idle at 700 for checking and with vac advance plugged off.

[Sagesurfing]

Initial timing set at 15 advance. I don’t use vacuum advance per engine manufacturer recs so it’s plugged

[geezer#99]

Yup!
I’ve seen that before.

Plugging in your vac advance for testing won’t hurt a thing. Nobody will know.
Give it a shot.

[RustyPile]

An 800 RPM base idle setting will cause run on (dieseling) when the ignition is turned off.. Unless you have a cam that has a lot of overlap, the idle speed should be about 600 - 650 at the most..

The air fuel mixture screws are for adjusting the idle mixtures not idle RPM.. You seem to be concentrating on the search for vacuum leaks.. There are a million things that will cause rough idle conditions.. These are only a few. Spark plugs fouling from overly rich condition. Faulty distributor cap, rotor, and/or plug wires.. .. Not to mention improperly adjusted valves and/or timing.. You will have a difficult time obtaining a suitable tune with not having vacuum control advance on the street. Never mind what the "specs" are.. Give the engine some settings that it likes...

You may not like the following: I'm just not a fan of Edelbrock carburetors. They, too often, cause the very problems you're having..

[Sagesurfing]

Thanks RustyPile. I am aware that 800 is high, however the idle is too rough when I lower it. You are correct about the air mixture screws, but adjusting the air to fuel ratio at idle will also impact idle rpm, so enriching the idle means readjusting the rpm back down. Spark plugs are new and gapped, wires are new MSD (been through plenty wires and spark plugs which haven’t resolved this issue.) I’ve adjusted valves. Distributor is new as well. Have replaced just cap and rotor in the past. I’ve addressed everything on your list and issue still happening. Keep in mind that it was happening on my previous engine as well, so that says a lot. As for vacuum advance… the engine runs fine. It’s just the idle that is the issue, and the idle would be getting anything if from a vacuum advance.

[Willshook]

You swapped the engine for a rough idle?

This sounds like a classic vacuum leak to me so I think you're on the right track. There are definitely many other potential issues worth a look, but I'd chase those all the way down. Double check for any un-plugged fittings and check EVERYTHING that has vacuum in it - note that even the transmission vacuum modulator and the brake booster can be a source of leaks. My usual approach is to plug everything at the source, then use a little starting fluid or carb cleaner to check the manifold and around the carb.

+1 on no vacuum advance on the street being a Bad Idea - giving up power and fuel economy...

[Sagesurfing]

So I did unplug the transmission and the vacuum lines on the intake, as well as the break booster on the carb. I plugged them all. I sprayed carb cleaner all around and got no change in idle. I’ve done this many times, never any change. About to unplug the PCV valve run a vacuum gauge there (plugging the breather on the other side) to see what internal info I get there. But tired of this nonsense lame idle.

[Sagesurfing]

And willshook, I replaced my engine cause I was having a bad hair day lol.
Jk. Have confidence that you’re not dealing with an idiot here. It was an old engine that had bad compression. My point was that the idle issue was occurring on the old one as well…

[geezer#99]

Your biggest vacuum loss is how open your throttle plate is to get an acceptable idle.

[Sagesurfing]

Can you tell me more Geezer? My understanding is that opening the idle screw (CW) will open the throttle plate and increase the idle. If that is the case, than closing the screw reducing idle RPM, and then feeding it more gas with the idle mixture screws would fix that, right? Currently my idle mixtures screws are out 2.5 turns, which seems like a lot to me. Can you explain further?

[HO455]

Did you reuse the same balancer and timing tap from the old engine on the new one?

If so I would verify the accuracy of the TDC marks. It's a good idea at this point anyway as it may help you to figure this out.

Here's a video by someone I respect on the subject that may help you troubleshoot the issue you're having.

https://youtu.be/DpU7pV4Li3A?si=qwlG1t8aHHILOliB

[dmjlambert]

Originally Posted by Sagesurfing

As for vacuum advance… the engine runs fine. It’s just the idle that is the issue, and the idle would be getting anything if from a vacuum advance.

I noticed something. I think you may have meant to say idle would not be getting anything from a vacuum advance. If that is what you meant to say, then you can try something. Give it vacuum advance at idle by connecting the vacuum advance to a manifold vacuum source instead of to the timed vacuum port. You should notice an increased idle when you connect the vacuum advance. In fact, that is how you can check it is a manifold vacuum port you connect the distributor to, the idle will increase if you do. If you connect to a timed port, you won't get any increase in idle. So when the distributor connected to manifold vacuum and you notice the increase in idle, turn down the idle screw. Enjoy your now smooth running and smooth idling engine.

Depending on how many degrees of advance your vacuum can gives you may need to set the advance to less, perhaps initial between 4 to 7. Take it for a drive with your new smooth idle and smooth engine, and see if you get knocking, then you'll know to turn the advance down. But leave the vacuum advance connected to manifold vacuum when you take it for a test drive.

[geezer#99]

Originally Posted by dmjlambert

I noticed something. I think you may have meant to say idle would not be getting anything from a vacuum advance. If that is what you meant to say, then you can try something. Give it vacuum advance at idle by connecting the vacuum advance to a manifold vacuum source instead of to the timed vacuum port. You should notice an increased idle when you connect the vacuum advance. In fact, that is how you can check it is a manifold vacuum port you connect the distributor to, the idle will increase if you do. If you connect to a timed port, you won't get any increase in idle. So when the distributor connected to manifold vacuum and you notice the increase in idle, turn down the idle screw. Enjoy your now smooth running and smooth idling engine.

Depending on how many degrees of advance your vacuum can gives you may need to set the advance to less, perhaps initial between 4 to 7. Take it for a drive with your new smooth idle and smooth engine, and see if you get knocking, then you'll know to turn the advance down. But leave the vacuum advance connected to manifold vacuum when you take it for a test drive.

Might I add to leave your initial alone and if needed limit your vac advance.