Carburetor upgrade/swap vacuum connections

[dmjlambert]

I am a carburetor newbie, and you may have seen my posts in previous days about the trouble I had with working on my old carburetor.

I have a 1969 CST/10 with 350 CI engine, and HEI distributor from an early 80's Chevy truck, and a TH400 transmission. I have a vacuum tree on the intake manifold with 2 ports on it, and I understand those are for the vacuum canister and the transmission.

I ordered a re-manufactured carburetor from National Carburetor. I told them what truck I have, and told them I would like an electric choke upgrade. The salesman said the carburetor with electric choke is a different carburetor because they can't swap out just the choke part, but he said they could outfit the electric choke carburetor with older-style linkages so it would work on my truck. I received the carburetor today and I am excited at the prospects of connecting it and getting my truck going.

Pictures below.

Old carburetor vacuum connections:
d1 to distributor
d2 (under fuel inlet) to PCV valve
d3 to air cleaner snorkel switch

New carburetor vacuum connections:
n1
n2
n3
n4
n5
n6
n7 this one is threaded but not sure what thread dimensions are, I was not able to figure out with a thread checker. A 1/2-20 thread checker will screw into it 2.5 turns before stopping. A slightly bigger picture of that port is below.

I don't know where to connect the distributor, PCV valve, and air cleaner snorkel switch on this new carburetor. Can you guys help? How do I plug or properly connect to n7?

[Mr Handy]

OK sense no one is helping I will try.
So first identify every thing that needs to be hooked up.

Distributor = manifold vac. N5 (one of the small ports on the carb, should be on the base. Some people will say to run ported vacuum but I disagree. You could Y where the hose is to the canister or the electric choke and have a shorter run to the dizzy)

Transmission = Manifold Vac. (This can go to the tree on the manifold behind the carb. or a manifold vacuum port on the carb.)

Power Brakes = Manifold Vac. (This would go to the big port on the carb, the big port on the manifold tree if you have it, or a threaded nipple to the hole on the back of the carb.)

PCV = Manifold Vac. N4(This would go to the big port on the front of the carb or to a nipple threaded into the hole on the back of the carb. Do Not hook it to the tree on the manifold)

Air cleaner = Manifold Vac. N5 with a Y so can also go to Dizzy (Make sure that the heat stove, the temp sensor in the cleaner, and the vacuum diaphragm on the snorkel all work. All I have seen have removed the guts and just have the door open all the time. If yours actually works that would be impressive to me. I double checked my service manual and it said Manifold Vac.)

I would buy a bunch of little rubber caps and plug all unused ports.
The hole at the back of the carb if not used needs to be plugged with a pipe plug. I believe it is a 1/4" pipe plug... I avoid the chrome one, try to find raw or stainless steel plug, they seal better. I like a drop of blue loctite over Teflon tape.

Just to be clear "Manifold Vacuum" comes from below the butterflies on the carb so that you get full vacuum at all times. "ported vacuum" comes from above the butterflies and only has vacuum when the butterflies are partially open. You may be able to search the model number on your carb and find a diagram that tells you which ports are manifold vacuum and which ports are ported vacuum. Most of the time the manifold vacuum ports come from the carb base and not the body.

[60case]

N1 canister purge, distributor ported vacuum N2 bowl vent cap it N3 air cleaner, distributor manifold vacuum n4 PCV N5 EGR cap it N6 choke vent cap it N7 power brake and trans manifold vacuum 1/4 pipe threads

[dmjlambert]

Thanks very much guys. Every little bit of insight helps me. I have been reading a tremendous amount online about the distributor vacuum being ported or manifold, and how the theory behind ported is to retard the timing and make the engine run hot at idle and burn more hydrocarbons instead of sending them into the atmosphere. There is a detailed article about it I found on a Corvette forum, and it included tips about how to use a small piece of paper held up to each port to figure if ported or manifold. So, I've got the first 2 types of emission-controls on this 1969 truck, PCV and ported vacuum timing advance control. I believe based on all I've read I will be using manifold vacuum on the distributor.

[Mike C]

Because the HEI was a product of the emission era, many need the vacuum cannister hooked to ported vacuum source so that it will generate enough total advance for max power/economy.

A dial back timing light or timing tab can be handy here as you try and figure what's best on your truck.

[dmjlambert]

I believe the port labeled d3 on the old carburetor was manifold vacuum, and that is where the distributor was connected when the timing was set and engine working well. On the new carburetor, I connected the distributor to the port labeled n3. With the engine running I was able to verify n3 is manifold vacuum, n1 and n5 are ported vacuum. I tested with a piece of paper, and it is not sucked to the n1 and n5 ports at all at idle but does hold the paper to the port when running higher than idle. n3 has suction all the time. n6 never has vacuum, I don't know what choke vent is used for. I capped all unused ports. I connected the choke to a power source that provides 12V with the key on. The engine is running well. Thanks guys for your help; I'm up and running now.

[dmjlambert]

Does anybody know where the bowl vent (n2 in above picture) went to in later model trucks? In a flooding situation did that vent gas somewhere to keep gas off of the engine? I wondered about this after seeing this post of dremmick's http://67-72chevytrucks.com/vboard/s...d.php?t=711767
I capped it on my truck, and now I'm wondering if I should put a hose on it and route the hose down to the road.