One or Two Vacuum Lines?

[Shoebox]

On my current 8ba engine build, I am using one of Jim(Bubba) Linders GM distributors which has vacuum (and centrifugal) advance. I also have a PCV setup, both needing manifold vacuum. My Offy manifold has one port back by the rear carb. Could I tap the dist. line into the PCV line, or would this mess up the signal to the distributor? I'm just looking to minimize clutter topside.


Thanks guys!

[aussie merc]

the one for the dissy goes to the carby dont know what carby your running but will need a ported vacume scorce not standard on 97s but you can buy a kit with easy instructions the one from the pvc goes to manifold

[flatjack9]

Read his first post, he has a converted GM distributor using manifold vacuum as a signal for the vacuum advance. You should be able to use that tap for both functions.

[aussie merc]

last time i checked a GM dissy the vac advance was ported to load supplement the centrifugal.

[69a]

If you are running a PCV valve you are putting a lot of airflow through it. It needs to be distributed evenly in the intake. there are several threads on here about that. That means either (assuming you are running a 2x2 manifold) drilling three more holes so everything is symmetrical. the way you have it you will have two cylinders running leaner than the rest. You can drill and tap a hole (one for each carb) on the under side of the intake. There are several options on the plumbing, you can have the PCV valve in the tappet valley or plumbed to outside.
As for your original question the consensus on here is to pipe the dist. advance can to inlet vacuum. Unless you are running a single carb with the correct port above the throttle plate. I chose to not run a vacuum on my GM disy.

[Charlie ny]

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My erzast vent system is a piece of 3/8 hose fitted to the fuel pump stand (8BA)
and thence fitted to the base of the air cleaner just above the clamping ring. I
understand the fumes go thru the carb and all that but it seems to work quite well.
I could not warm up to the idea of messing with inches of engine vac and a pcv valve.

Charlie ny

[Gene1949]

Originally Posted by Shoebox

On my current 8ba engine build, I am using one of Jim(Bubba) Linders GM distributors which has vacuum (and centrifugal) advance. I also have a PCV setup, both needing manifold vacuum. My Offy manifold has one port back by the rear carb. Could I tap the dist. line into the PCV line, or would this mess up the signal to the distributor? I'm just looking to minimize clutter topside.


Thanks guys!

I faced the same dilemma and a wiser head here was of the opinion that the actions of the PCV would influence the vacuum advance can.

When you think about it the PCV line is pretty small in comparison to the overall volume of the manifold plenum. By drawing from 2 different locations the overall effect would be to reduce the effects/actions of the PCV on the advance can.

Believe me, moving your can to manifold (vs timed) source provides a drastic improvement in low end cooling and overall drivability.

Not sure what intake you have but my Edmonds 4 bbl intake is totally divided (left bank/right bank) so I had to come up with a spacer to drill into and tap both ends.

[Shoebox]

Well, some differing viewpoints here. My GM dist needs manifold vacuum (I learned that after buying the ported base for my 97. I have a PCV setup with the valve going in the hole where the road draft pipe would go, which also calls for manifold vacuum. Glad I'm converting to electric wipers, or I'd have a third device needing manifold vacuum. Think I'm going to T into the PCV hose for the dist and see how that works. Any more thoughts?


Thanks guys!

[69a]

You can "tee" into the vacuum line for your dist. just keep as close as possible to the intake, because it is a dead end circuit it will supply intake vacuum to the dist. near enough.
You obviously do not understand how a PCV system works. You need to distribute the air from the crankcase evenly into the inlet manifold. You also need a decent flow to the PCV valve to ensure it pulls the slug all the way to the end. If it doesn't that will effect the idle circuit.
Check out the previous threads on here for PCV systems, you may learn something.

[aussie merc]

so only one way to resolve this and thats contact bubba he will be able to say exactly where the dissy vac line needs to be hooked up [as he modified it] i havent dealt with these MODIFIED dissys so they could well be hooked directly to manifold vac love to see the answer

[Mart]

Without anything more to go on than gut feeling I would say use two lines. One for the PCV and one for the distributor.

Mart.

[Charlie ny]

I make these GM distributors....a lot of them....do not 'T'. Best results....source vac
for the distributor

from base of carb or use 'South Wind' style vac plate bet the intake and carb. I hope
Flat Jack Meyer will rejoin this thread to get his take on reduced vac with multi carbs.
In my experience the GM sbc/8BA conversion distributors work best with a 2 or 4 brl carb. when utilizing the vac can. Mostly customers with the notable exception of Jack
opt out of using the can and go 100% mechanical adv.
Charlie ny

[Shoebox]

Originally Posted by Charlie ny

I make these GM distributors....a lot of them....do not 'T'. Best results....source vac
for the distributor

from base of carb or use 'South Wind' style vac plate bet the intake and carb. I hope
Flat Jack Meyer will rejoin this thread to get his take on reduced vac with multi carbs.
In my experience the GM sbc/8BA conversion distributors work best with a 2 or 4 brl carb. when utilizing the vac can. Mostly customers with the notable exception of Jack
opt out of using the can and go 100% mechanical adv.
Charlie ny

Charlie, I know you have done a ton of these GM distributors, so per your recommendation I will not T the vac line, I have options.

I have a pristine Mallory flat top flathead distributor that is centrifugal only, then I can use my pcv setup or I can use the original road draft tube and use the GM dist. with manifold vac. I can leave this issue till almost the last.

[Shoebox]

Originally Posted by 69a

You can "tee" into the vacuum line for your dist. just keep as close as possible to the intake, because it is a dead end circuit it will supply intake vacuum to the dist. near enough.
You obviously do not understand how a PCV system works. You need to distribute the air from the crankcase evenly into the inlet manifold. You also need a decent flow to the PCV valve to ensure it pulls the slug all the way to the end. If it doesn't that will effect the idle circuit.
Check out the previous threads on here for PCV systems, you may learn something.

Thank you for the information, however I feel your are rude.

[Flathead Fever]

A more modern engine like a Ford 289/302 would have about a 3/8" "fuel rated" hose not vacuum tubing going to the back of a spacer plate between the carb and intake or directly to a fitting on the back of the carburetor as the vacuum source. The PCV fumes and oxygen get mixed in evenly when you have the centralized carburetor so you can adjust the Idle air fuel mixture and jet the carburetor to compensate for the valve. It's a good idea because it prevents sludge buildup which happens when the blowby fumes eventually create an acid which contaminates the oil, reducing its lubricating qualities and also causing that sludge buildup. Engines did not have PCV systems until around 1963 when the entire automotive industry added them. It's not necessary to have a PCV system if you change your oil often, something like every 1000 miles. If you have an old-tired-engine with a lot of blowby the PCV system will help the oil last longer. I've never installed a PCV valve on a flathead because I just don't drive my cars enough to worry about it., I just keep the clean. The problem I'm often thought about with multiple carburetors is where to install a PCV valve so be you're not leaning out cylinders.

Most factory PCV systems do not have anything tee'd into the "manifold vacuum" line to the PCV. Maybe because the PCV hose is drawing in fuel vapors which deteriorate non-fuel rated hose. Vacuum hose is not rated for fuel vapors, it starts deteriorating quickly if you try use it on a PCV valve. And you probably do not want your vacuum source having crankcase vapors inside the hose because it would deteriorate the rubber diaphragm inside the vacuum advance or vacuum operated smog components on the later engines.

[69a]

Originally Posted by Shoebox

Thank you for the information, however I feel your are rude.

Re PVC valve to intake plumbing an appropriate quote someone uses on here comes to mind. "I can explain it to you, but I can't understand it for you"
If you fix your PCV issue you will have a spare port for your dist. vacuum line.

When a PCV valve is fitted correctly there is virtually no flow through it with full vacuum (idle or over-run)

[blucar]

A lot of dialog regarding a very simple subject, however, I think the majority of the comments missed the point..
A PCV system needs a dedicated vacuum supple, the PCV has a valve with-in that snaps closed upon acceleration allowing vacuum to flow for the vacuum advance, once acceleration is completed the valve snaps open allowing the crank case vapors to be pulled into the intake and burnt off.

[69a]

The PCV valve is effectively closed at both ends. Closed at full vacuum, and closed with a spring(at the opposite end) at zero vacuum, this is to prevent an intake backfire from reaching the crankcase. My point to the OP that a "tee" fitted close to the intake will be less detrimental than pouring all the crankcase fumes through one port, therefore running lean on 2 cylinders the whole time it is anywhere between full vacuum and zero vacuum. (his intake is an "Offy" I'm assuming it's a 2x2). I suppose you could drive with either full throttle or none the whole time, this would limit the unequal mixture problem, But the PCV valve would be closed 90% of the time. And his vaccum can on his dist. would still work fine if teed in close to manifold.

[flatjack9]

Originally Posted by Charlie ny

I make these GM distributors....a lot of them....do not 'T'. Best results....source vac
for the distributor

from base of carb or use 'South Wind' style vac plate bet the intake and carb. I hope
Flat Jack Meyer will rejoin this thread to get his take on reduced vac with multi carbs.
In my experience the GM sbc/8BA conversion distributors work best with a 2 or 4 brl carb. when utilizing the vac can. Mostly customers with the notable exception of Jack
opt out of using the can and go 100% mechanical adv.
Charlie ny

The "South Wind" style vac plate is ideal for this situation. I have done it many times. I should have thought of that in my initial post on this subject. So called reduced vacuum is not a problem. Running at part throttle, vacuum will be the same whether with one or two carbs. As Charlie has mentioned, I am a huge proponent of vacuum advance on any street engine.

[69a]

"South Wind" vacuum plate suggested by Charlie ny and flatjack9 would be a great solution for the OP you must fit one under each carburetor and connect them ALL to the PCV valve. That will leave him the factory 1/8 NPT to plumb his distributor!