Vacuum Advance - Which port? Nailhead 401

Little things keep raising their heads. Stuff that I was always going to deal with later. Well, later has arrived.

I'm running a stock 401 with an Offy 3x2 intake. It has 1 vacuum port from factory. I was all set to use it for my PCV when I spotted the vacuum advance on my dizzy.

Can I plumb the vacuum advance to the same port as the PCV? I don't really want to drill another one if I don't need to.

I'm thinking that the vacuum signal should be the same wherever I plumb it but I also see the crank case as a big vacuum leak that could impact my vacuum advance.

Tell me, smart people.

Thanks, Pete

 

PORTED Vacuum, from ABOVE a throttle plate

 

Yep, thanks. Makes sense. Next decision. Do I really want to drill one of my Stromberg bases?... Don't answer that...rhetorical question.

Pete

 

Do you have a threaded hole tapped directly into the manifold for vacuum?

Where is the hose from the PCV going?
To the back of the carb flange on the manifold or into a carb base?

I don't see a problem with T'ing the vacuum advance line into the PCV vacuum fitting.
It's a small line and the vac advance setup is a closed system.

Don't take the easy way out and do away with vacuum advance.
Do that and you'll more than likely end up with an overheating engine at idle and in slow traffic.

Ported vacuum usually leads to overheating.

GM designed the ported vacuum setup to make the engine run hot at low speeds for smog purposes.

 

yep dont use ported that a mid 70's GM with HEI thing. manifold vacuum is what you want. Were any of the carb stud holes drilled all the way theough? use a hollow carb stud

 

You want manifold vacuum. Just because the port is above the ****erflies does not mean it is "ported" vacuum. The small hose ****** on the right carb body and the threaded hole in the same location on the left carb body is the factory vacuum advance connection. They look like they would be above the ****erflies but they are not. The port goes in about a 1/4" and then turns down coming out below the ****erflies in the base. It is manifold vacuum even though it enters the carb above the ****erflies. All of the Delco-Remy distributors from those years work the same way..Olds, Buick, Pont. and Chevy. The Delco-Remy dist in my Studebaker is the same way.

Any port in the manifold will work.



Read what a GM engineer that actually worked on vacuum advance wrote about the subject.


As far as the PCV goes, My offy catalog shows a vacuum port located in the front runner of the intake. I'd use this port for the vacuum advance. Hooking a PCV valve to this port will lean out that cylinder. A vacuum advance does not allow air into the intake like the PCV does. If you check all of the factory PCV hook ups, you will find them right near the base of the carb so that the extra air added below the carb will get homogenized to all 8 cylinders.

I'd drill and tap the center carb base riser in the intake if there is enough room. If not, then I'd make 3 aluminum 1/2" spacers under the carbs with the center one tapped for your PCV system.

 

I was a firm believer in manifold vacuum for timing advance, used to tell people why it was best, used to post a script explaining all about it, argued the whys and wherefores several times.

BUT, I then had a car that had a continual low rpm misfire/fart and after spending loads of time, a new intake, new carb, fitting a spare distributor, coil etc, it eventually turned out to be the manifold vacuum connection that was causing the problem.

Apparently some types of distributors do not work well on the heavy vacuum a direct manifold connection gives, and mine was one of them. Under load the timing would flutter about a bit, causing the erratic running. I put it on ported vacuum, reset the timing, and it ran perfectly ever afterwards.

 

BINGO and it also has to do with which vac can that is installed ..... An easy almost always condition points= manifold and HEI= ported and yes they can be modified either way HOWEVER that is a good general statement

 

Oh ****, now I have opinions. Not to discount anything anybody has written, but I have read some of Tommy's older posts about PCVs etc and he seems to make more sense than most. Why can't we all just agree?

Thanks to all. I have some more thinking, looking and planning to do...and a bit of doing too.

Pete

 

Just read that article by the GM engineer too, Tommy. That was good. I am pretty dumb when it comes to this kind of stuff and even I could understand it (part 1 anyway).

I think I'll have a look at drilling and tapping the underside of the intake under the centre carb. It's probably the easiest spot to do it and tidy too.

Pete

 

Differing opinions due to different cars in most cases and striving for different things in other cases.

I'm surprised that Morrisman's probs were cured by changing the vacuum advance timing source.
Can't argue with success.

Fwiw - get yourself a pair of fairly well matched vacuum gauges, put one on manifold vacuum and the other on ported vacuum then take the car for a drive.

You'll see some interesting things....

 

Thanks for the info Tommy, re-conected the vacuum to my dizzy, that my
" machanic mate " blanked off a while ago, and the 331 Cad is running a
treat.
Thanks for putting it in english also, Martin.

 

My pleasure. Your gas mileage should be better now. It's one of those internet myths that keeps getting repeated. Other than the 49-56 Ford, I know of no pre-smog factory carbs that have a "ported vacuum" connection.