My 390 only has 8 psi vacuum at idle been all over it with WD40 and can find no leaks. When I disconnect the EGR it drops to 5 I checked the idle screw on the Holley 10554B vacuum didn't change Both screws all the way in engine died Both in 11/2 turns Timing is at 22 Pretty high but that seems what it likes right now Searched past threads because this topic is talked about a lot but nothing seem to be my exact issue The issue is the engine stumbles on start off and will idle fine for awhile then load up and die I also installed a fuel press regulator so should have 5 psi and changed fuel filters

 

Do you have another vehicle that you can use the vacuum gauge on, just to verify that the gauge is working properly? That would be my first step.
Next would be to verify the timing marks. Things get swapped around sometimes (maybe yours has not and it’s all original.) but even things like the damper ring can slip overtime.
Is it the original damper?

Anything else been done to the engine over the time you’ve owned it? Non-stock camshaft will definitely lower your vacuum.
are you the original owner? Or is this truck fairly new to you?

Also, when checking overall vacuum, are you using a carburetor port, or a manifold port? Sometimes carburetor ports can be misleading.

 

Vac gauge is new I know that doesn't always mean good but would assume so Cant speak on the cam Ive had the truck a year and don't have any info on it like that Sounds like a stock or mild cam The only thing was relayed to me was the engine has 60k on an overhaul No way to confirm I T'd the vac gauge in to the line running to the EGR

 

Is this a permanent vacuum gauge in the cab? Or just a temporary one for diagnosing stuff under the hood?
Personally I would run it directly to an unused, or uncapped port under the hood if it was just temporary. Or if a permanent – mounted gauge, I would Tee into something else besides the EGR.
What year is your truck again? Is the EGR plugged directly to intake vacuum? Or is it running through all sorts of vacuum modulator stuff? Like a 78 or 79 would have been?

 

Like 1Ton said the only way you'll know on an engine that you've never verified the timing marks on the harmonic balancer are correct is with a piston stop in the no. 1 spark plug hole.. Only then can you say with confidence "My timing is set at X*". The port you used is the same vacuum tree that the EGR and power brake booster connect too ?

 

the truck is a 74 The first pic is how its set up and second is how I set up my temporary test gauge

 

Is that a Chinese carburetor? It looks like a copy of a Holley. If it has a power valve like a Holley it could be blown out.
That will cause a “load up” condition. How many turns from fully closed throttle plates to sustainable idle? Anything more than 1-1.5 turns is usually too much. Try putting your hands over the openings on the valve covers and see if you feel a vacuum which will indicate a lower intake manifold leak into the crankcase.

 

To do your vacuum gauge readings you'll pull right from the intake where the vacuum tree is at is a good place. Where your at will give you an inaccurate test reading.
Recheck your manual choke to be sure it is completely open when you set it there from in the cab.

 

Ok that is not the EGR port but the vacuum advance port on the dist. you have the vacuum gauge teed into and it looks to be going to the carb that would be ported vacuum.
If you know that is not the EGR valve then the EGR should not have vacuum to it either.
Ported vacuum only has vacuum above idle speed and why you may have a low reading if the idle speed is to high.
Manifold vacuum is what the motor sees all the time and is high at idle and goes lower under load.

You need to find a port on the intake manifold that is manifold vacuum and hook the gauge there for the right vacuum rading.

Now I see / hear something wrong now that I see where you have the gauge hooked to.
You should not have vacuum to the dist. advance when at idle.
What is the idle speed set to? If to high you can have vacuum to ported vacuum and the mechanical timing can also start to kick in.
Set the speed to factory, vacuum hose off the dist. and set timing to say 10* BTDC as a start.
You may need to reset the idle speed and see if you still have vacuum on the hose to the dist.

Now you can adjust the carb and timing using a vacuum gauge hooked to MANIFOLD vacuum.
You still need a tach for RPM to set idle speed.
Check / set timing with a light or highest vacuum reading on the gauge, no vacuum hooked to dist.
When using the gauge for timing if you get pinging or kick back on hot starts retard timing till both stop and note vacuum reading as your "set to point".

Once timing is set and the idle speed set back to spec using the vacuum gauge adjust the idle mix screws a little on each one for the highest reading.
They should both be set the same amount in / out when done.
See how it runs and let us know and we can go from there.
Dave ----

 

It says Holley on the side of the carb did what you suggested and no vacuum in the case. It idles at 1.5 turns out

 

What is 1.5 turns out the idle mix screws?
The may be a good starting point after a rebuild but may not be the best for the motor.

You need to start just like you did a full tune up for setting everything back to spec.
Motor up to temp, choke open all the way and idle speed set to factory spec.
Pull the vacuum hose off the dist. and set the timing. You may need to go back and reset the idle speed.
Then adjust the carb idle mixing screws turning each one a little at a time in or out to get the highest RPM or vacuum reading.
Readjust the idle speed and take for a test drive.

As you can see EVERYTHING is set off the IDLE SPEED and why you need to keep this in check.
BTW if you have everything set back to factory tune up spec and the manifold vacuum reading is still low and the motor dose not have a big cam installed the timing chain marks could be off when the cam was installed.
Dave ----

 

You engine wants 22° of timing at idle because it's lean, are you sure it's "loading up" ?

 

Sorry about the delayed response had to go to work. My hitch went a little longer this time. So your right it is lean so not loading up but still dying after idling perfectly for awhile. It is probably lean due to that huge crack in the exhaust manifold. Would you concur?

 

You vacuum gauge should not be plumbed into the distributor advance can vacuum. I see 7 inches vacuum on the gauge. Ported vacuum only opens up above idle.

Try running the engine without that fuel pressure regulator too. That looks like the two FPRs I first tried, both killed the engine quickly after start up.

 

Maybe, but probably not.
Got a pic of the crack we can see? I've seen/experienced a lot of cracked manifolds over the years, and while they all had some minor effect or another, none of them ever changed the mixture so badly that they caused the engine not to run or vacuum to drop precipitously.
It's certainly not helping anything, but if it's just a crack it's not likely to cause the whole issue unless your carb is already jetted way too lean to begin with.
I wouldn't think anyway...

But again, I've never experienced a really big break in an exhaust manifold. So can't say about yours.
But header gaskets leak all the time, sometimes badly, and while they also effect tune, they do not cause a large drop in vacuum or loss of idle.

Did you change the location of the vacuum gauge to the manifold yet?
Have you tried adjusting the pressure regulator?
Have you tested fuel pressure with a gauge yet?

Paul