Rough idle- low vacuum

[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!

[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

[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.