WESCH

Hi
I was thinking about my vacuum advance hose connection to my 68 big block.
The HOLLEY 4150 , 650 CFM double pumper mechanical 2nd has one vacuum port on the back at the lower plate, this port connects to the ignition distributor vacuum advance and is also teed off to the left hand ( drivers side ) cylinder head cover vent.
Is this so OK ? Doesn't the blocks vent interfere with the distributor vacuum ?
The carburator also has a smaller vacuum port at it's front lower plate. Should I connect the distributor to this port ?
Thanks. Gunther :seeya


[Modified by WESCH, 2:11 PM 1/19/2003]

Smokehouse69

Gunther,
Normally the vacuum advance line is connected to a ported vacuum source. This is a source that doesn't have vacuum on it until the throttle blades are open, like at cruise. As a rule at normal idle you don't want a lot of advance on your distributor. The exception to this rule is with cars that have a very big cam, and don't idle very smooth and have a very small manifold vacuum at idle (7-8 inhg) then hooking to straight manifold vacuum will put in a little advance at idle and smooth out the idle a little.
I'm not sure which port on your Holley is a ported source, on my Holley there are two ports, one right over the other. The bottom one appears to be a full vacuum source, the one on top appears to originate above the butterfly, so it is possibly a ported source.
The only way to tell for sure is to use a vacuum gauge on each of the ports, while the car is at idle. The one that is not pulling a vacuum or has lower than manifold vacuum is probably a ported source.
I hope this helps you out.
Good Luck!

Vetterodder

Early 4150's only had the lower, full manifold vacuum, port on the front passenger side of the carb while the later versions also had a ported source above it. Those ports are the ones intended for vacuum advance but I don't know that there'd be a problem with it as it is on your car. The large outlet at the rear is for PCV and/or power brakes.

WESCH

Hi
Thanks, but I'm still confused.
See the second pic, this is the carb port at the back ( driving direction view ).
Both, the distributor vacuum and the engine block vent from the left hand ( drivers side ) valve cover are connected to this port.
My engione is a 427 CI block with L71 heads and original L71 solid lifter cam.
I will install a gauge to this port to see what it is.
If this is the port , the power barkes would connect to, it should be full vacuum ( not ported ).
The first pic shows the plugged front carb port, which is much smaller in diameter.
What do you think ?
Thanks Gunther




[Modified by WESCH, 12:13 PM 1/20/2003]

Smokehouse69

Gunther, a ported vacuum source is one that is above the throttle blades. If I look down the the throat of the front two barrels of my Holley, I can see a tiny hole in each throttle bore right above the throttle blades. There is a vacuum port that is apparently connected to these two holes. The port that these holes are connected to will not have any appreciable vacuum on them while the car is idling. Once the throttle blades are opened up there should be vacuum on the port that leads to these holes.
The only way you can really tell if a vacuum port is ported or unported, unless you have the carb off the car and can trace the route, is to put a vacuum gauge on it while the car is running. A ported source will not show much vacuum on it while the car is idling, while an unported source (vacuum brake booster) will have close to manifold vacuum on it.
I know there are plenty of guys here on the forum who are running the same Holley carb you are, so they should be able to tell you which port is a ported vacuum source.
I wish I could have provided better information for you, perhaps if you re-ask the question, with the following title "Ported vacuum port on a Holley 4XXX carb?" Maybe someone with the same carb will tell you where they have their vacuum advance hooked up.
Good Luck!

Vetterodder

The capped port on the front corner of the carb in your picture is intended for full manifold vacuum advance (which is what you have as it's plumbed now). If you're carb has another port above that on the metering body, that one's for ported vacuum. If it doesn't have one, your carb is a pre-smog era unit and ported vacuum wasn't used.

WESCH

Thanks.
Only one ea port on each side of this carb.
So both ports should be full vacuum ( non ported ).
Shouldn't I connect the distributor vacuum advance hose to the caped front port in order to keep it away from the block vent and oil fumes and also to assure non disturbed vacuum going to the advance ?
Sorry, I keep on asking the same question.
Again, thanks a lot so far.
Gunther :seeya

WESCH

Hello Smokehouse69

Are you sure about your first statement, that the vacuum advance should be connected to a ported source ?
This would mean that the vacuum advance would work with the flyway advance in conjunction adding up.
I'm a bit lost now on understanding how both this advance ignitions are suppost to work.
At what RPM does which advance move how much ?
Ups, I just confused myself.
Gunther :crazy:

jimtreber

I think I see what you're getting at. Are you concerned that the crankcase ventilation system is bleeding off vacuum, robbing the vacuum advance of its full amount? I would feel the same way. Check that unused front port to see if you have vacuum at idle. If you do, run your distributor advance from there. Vacuum advances using ported vacuum didn't start until the first smog year, 1971.

Vetterodder

Ported vacuum works the same as manifold vacuum except that it's not active until the throttles are opened (or Bubba has adjusted your idle). I'd move it to the front port but, functionally, there should be no difference whether you leave it as it is or if you move it to the "proper" port at the front on the carb. The flow is towards the manifold (because it's a vacuum) and away from the vacuum canister but, over time, enough oil might find it's way into the vacuum canister while the engine was off to cause a problem. There is certain amount of vacuum available and it doesn't matter which port it's drawn through as long as none of the ports have a leak or a restriction.

WESCH

Thanks a lot.
I will check both ports with a gauge as soon as my engine is running again.
If the front port is the same as the aft, I will move the distributor advance hose to the front.
I already had to replace the vacuum advance due to blown membran and oil was evident in there.
Thanks for the help.
Gunther :cheers: