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.

[HO455]

Do any of your spark plugs show signs of oil?

[dmjlambert]

Originally Posted by geezer#99

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

Yes, perhaps. But that's hotrodding the distributor IMO and something to explore later, so I advise get it silky smooth first, HP later.

[Sagesurfing]

HO455 the spark plugs look perfect. And the trucks runs well, though gas mileage is low at 8-9mpg. Should get more from a newer crate engine.
The issue also become more pronounced after the engine warms up and then sits for a minute, which makes me suspect vacuum leak somewhere due to expansion. But I just can’t seem to find a source.

[72c20customcamper]

If you had the same problem with two engines I suspect something else is causing it . How old is the BluePrint motor ? I installed one in my 72 this spring have 5k miles on it not one issue .

Also the engine was run on a dyno so no fiddling with the carb or distributor should be needed other than maybe jetting for altitude. Mine was pretty much plug and play

[67carry-all]

"The only parts I didn’t swap were the intake manifold..."

Just throwing this out there - if you're having the same issue between the two engines, consider parts that were carried over, maybe look for a cracked/warped intake manifold. Good luck!

[HO455]

Originally Posted by Sagesurfing

HO455 the spark plugs look perfect. And the trucks runs well, though gas mileage is low at 8-9mpg. Should get more from a newer crate engine.
The issue also become more pronounced after the engine warms up and then sits for a minute, which makes me suspect vacuum leak somewhere due to expansion. But I just can’t seem to find a source.

The only parts I didn’t swap were the intake manifold..."

Just throwing this out there - if you're having the same issue between the two engines, consider parts that were carried over, maybe look for a cracked/warped intake manifold.


That was the driver behind my question about the plugs. It's possible that there is a porous or cracked area in the intake manifold. If it was in the valley it is likely that you would see oil on the plugs. It could also be between the exhaust cross over and intake port.

Are you running a factory cast iron manifold or an aluminum aftermarket one?

Are you using a carb adapter to bolt the 1406 to a spreadbore manifold?

As I mentioned earlier a bad balancer would cause you to set your timing incorrectly. Try setting the timing for best idle (Ignoring the actual numbers.) and then try adjusting the Idle set point and mixture.

[Willshook]

It's unusual to have an intake that has porosity or cracks - not unheard of, but unusual.

What's not unusual at all is that the manifold has been cut over the years - when blocks are decked then intakes get cut to match. That can create a leak at the bottom of the intake runners. It wouldn't be detectable from the outside. I've seen it a fair bit, but I always check for this when installing a manifold. Sometimes people also use the thin FelPro gaskets on these manifolds when they should be using the thicker ones. The same situation can happen in reverse where the intake was new...but the block and heads were surfaced with pretty deep cuts...also creating an angle mis-match and a leak.

[72c20customcamper]

Originally Posted by 67carry-all

"The only parts I didn’t swap were the intake manifold..."

Just throwing this out there - if you're having the same issue between the two engines, consider parts that were carried over, maybe look for a cracked/warped intake manifold. Good luck!

When he said he didn’t switch over the intake I took it as he left the intake that came with the BluePrint motor on it . The only items I had to install on the blueprint motor ,water pump,balancer ,flex plate ,fuel pump ,power steering and alt .

Engine came with carb intake and distributor installed.

The OP didn’t specify if it was a short block or dressed motor

[Dashman]

What are you using for carburetor to manifold gaskets?

I've used a 1/4" phenolic carburetor gasket (*not gasket, spacer) with an Edelbrock 1406 carburetor because the fuel in the centralized bowl was vaporizing. If one thinks about it, the Quadra Jets came with thick gaskets for this reason.

[72SB]

OP. I have a BPE and the Edelbrock 1406

You absolutely need VA hooked up to manifold vs timed vacuum on carb. You will then need to back off "base" timing as it will provide some timing advance at idle. Yes I know the Edelbrock 411 says use timed...they have to say it, emission thing.

You need to nail timing before final carb adjustments, or you will be chasing your tail. Assuming your damper timing marks ae correct in relationship to your pointer (read I had to get a different pointer for my BPE than my original 327). Pointer is based on what size damper you have. My BPE came without manifold, carb, dizzy and accessories.

You want 20-24 degrees of timing at idle WITH VA. Depending on how much VA your distributor VA provides at X rpm, you adjust "base" timing accordingly. Lets say the VA provides 10-12 degrees of advance at idle so you would then want 10-12 degrees of "base" timing (at idle no VA). Yes its a deeper conversation on controlling how much and when VA applies and stops but for base settings, shoot for 20-24 degrees of advance, with VA connected to manifold vacuum on carb (lower port).

Then adjust the idle mix on carb. You want to be closer to 1 1/2 turns out. Adjust either way from there to achieve the most engine vacuum. Then adjust idle. If automatic, likely about 750 as putting it in ger will lower idle from trans load. Manual trans, 650 should work but just adjust accordingly

You just don't have enough timing at idle without VA.

nailing the VA involves using an adjustable VA and a detent plate to restrict how much it applies. Different conversation. You can buy adjustable VA but detent plate, not any more. Crane used to sell the kit. Home made version of stop plate is doable

Understand not using VA on a street car reduces gas milage, loads up plugs, motor runs rich at part throttle use (read 95% of normal drivin conditions)