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Distributor vacuum 78 302

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Old 09-09-2017, 12:09 AM
SSStang SSStang is offline
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Distributor vacuum 78 302

I have a 78 with a 302.
1. The vacuum line that connects to the distributor connect to where?

2. Should there be vacuum at idle to the distributor?
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Old 09-09-2017, 12:16 AM
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75BigBlock 75BigBlock is offline
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I'm not sure on the differences from 77 to 78, but my 77 has the vacuum from the distributor connected to the front lower left vacuum port on my 2100 carb. As long as your vacuum advance isn't broken like mine used to be, it should work fine. Mine was having issues at one point and it would die at idle. I unplugged the vacuum line and it ran fine until I got things replaced.
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Old 09-09-2017, 12:18 AM
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75BigBlock 75BigBlock is offline
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Also, my 2100 didn't come with the truck, it came with a 2150, but I would imagine it's similar as well. Front lower left if you have the hood open and you're standing in front of your truck looking at the carb.
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Old 09-09-2017, 12:32 AM
SSStang SSStang is offline
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I just replaced the distributor because it was bad along with plugs cap rotor and wires.
Right now it gets vacuum from a port on the back of the engine. I don't have a port for vacuum on the front of the carb. It's a 2150 carb I think.




This was before rebuild
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Old 09-09-2017, 12:40 AM
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Here's a picture of one upside down. Mine is plugged in at the port right under the choke assembly on the right side of the picture. Number 5.

https://www.google.com/search?q=2150...zZng7MdhkNo6M:
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Old 09-09-2017, 01:30 AM
1TonBasecamp 1TonBasecamp is offline
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From the factory I think they all came with a temperature controlled setup with multiple hoses and full manifold vacuum linked through at least one, if not two separate TCS' (temperature controlled switches) I think they're called? One was usually right there in the front on the water neck on a 302.
Not sure if the '78's still had the dual-diaphragm vacuum advance or not.

But in the simple basic form, with just a hose straight to the carburetor, you would use ported vacuum. So no, there would be zero vacuum at idle in that case.
And if it's still a dual-diaphragm advance can, you use only the front fitting and leave the rear one open.

Paul
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Old 09-09-2017, 10:51 AM
SSStang SSStang is offline
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Ok looks like it is broken off?


Right side next to the nut
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Old 09-09-2017, 11:13 AM
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You can connect it to ported, or directly to manifold vacuum if you want. Keep in mind it will pull in a lot of advance at idle, if you look at it with a timing light. Maybe 30 or 35 say. This is no problem, as the engine is not under any load in neutral or idle. As soon as the engine is accelerated the vacuum advance goes away, until the speed/load levels off again. Have to experiment a bit, whatever runs best. Manifold connection and lots of idle advance helps keep engine temps a lot cooler, if you live where it gets real hot or stop and go driving this may work well for you.

Sometimes it's difficult to achieve a steady idle, or stalling issues with A/T though. Keep in mind there is no difference in the amount of ignition advance cruising down the highway or at part throttle acceleration etc, same either connection. Just at idle. So whatever works for you.

The important thing though first, is to see what the total mechanical advance tops out at (with vacuum advance disconnected and plugged) and adjust for maximum advance that your combo will tolerate. Take some test drives and full throttle acceleration etc.

When you are satisfied there, then re-connect and experiment with the vacuum advance only - connect at either manifold or ported and see what you think. They are adjustable. Start with it pegged out on the high side and back off till there is no part throttle or steady cruise rattle. Usually the rule of thumb is stock basic engine : ported, hopped up performance engine, big carb, hot cam : manifold.
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Old 09-18-2017, 09:50 AM
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Ok finally had some time to work in he truck. The port on the were I though was for ported vacuum to the distributor gets no vacuum when Reving the engine.
The truck idles and has good throttle response with manifold vacuum. When I go to drive it misses and pings and no power.
I want to run ported vacuum but not sure where to connect it? Only place I see to connect it is if I put a T here.


I got the truck not running.
The line on the bottom left
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Old 09-18-2017, 11:24 AM
KULTULZ KULTULZ is offline
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Originally Posted by SSStang View Post

Ok finally had some time to work in he truck. The port on the were I though was for ported vacuum to the distributor gets no vacuum when Reving the engine.

The truck idles and has good throttle response with manifold vacuum. When I go to drive it misses and pings and no power.

I want to run ported vacuum but not sure where to connect it? Only place I see to connect it is if I put a T here.



I got the truck not running.
The line on the bottom left
There should be (may - depending on model of 2150) a Spark Port is @ the front of the choke asm (the photo you provided in another post shows a hole, the fitting may be missing. Is there vacuum there when you put your finger over it?



You are not referring to the choke pull-off vacuum source are you as that is manifold vacuum

Look @ this diagram. The Spark Port is #5-

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Old 09-18-2017, 11:50 AM
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I will have to check agian but the spark port had no vacuum at idle and no vacuum when opening the throttle.

Yes I was referring to the choke pull down.
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Old 09-18-2017, 12:06 PM
KULTULZ KULTULZ is offline
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I will have to check agian but the spark port had no vacuum at idle and no vacuum when opening the throttle.

Yes I was referring to the choke pull down.
Even if controlled through a PVS, ported vacuum has to be there somewhere.

What did you do, roll that carb off a cliff...
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Old 09-18-2017, 12:56 PM
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Remember the mechanical advance is what you're mostly concerned with. Don't worry about vacuum advance unless and until the distributor mechanical curve is setup. This takes trial and error and a few test drives. Then re-connect the vacuum can and tune that with an allen wrench.

There won't be any vacuum at the instant you goose the throttle at a ported connection, only when it starts to level off at a high RPM. You can see this when a mechanic's vacuum gauge is connected to a source of constant manifold vacuum. A stock engine will pull about 18" steady needle at Sea level. When throttle is quickly winged the vacuum will instantly drop to almost nothing, maybe 5" and then quickly rebound to maybe 25" (if the rings and compression is good) and settle back down to a steady 18".

Here's an overview that's real good about the basic concepts of mechanical distributor timing versus vacuum advance and strategies for optimum tuning. It's totally worth taking the time to understand this and you'll thank yourself every time you drive it. So will your wallet, best performance also means best economy.

http://www.squarebirds.org/simplycon...istributor.htm
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Old 09-20-2017, 11:24 AM
SSStang SSStang is offline
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Ok trying to learn here.
Wanna to try run the truck on ported vacuum. Took carb back off and my ported vacuum looks sealed off? Port on back of carb is sealed off too.


What are other options for ported vacuum?
Here are some pics



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Old 09-20-2017, 03:15 PM
1TonBasecamp 1TonBasecamp is offline
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Maybe a PO did that, maybe a rebuilder did it on request, or maybe it was that way from the factory for a different application or model year.
It's possible that the big port is capped off because the spacer originally used had it's own vacuum port.
Several possibilities, none of which I know for sure. Hopefully someone knows the differences, but you definitely want a common 3/8" port for the PCV valve, a single 3/8" port off of at least one runner for the vacuum brake booster (if applicable) and a small ported one for the distributor unless you're going to run every bit of the original "smog stuff" all present and functioning.

Not sure what the best way to proceed would be. Either find those ports elsewhere, or open those you have back up. Maybe one of the carburetor experts will chime in with what can be done.

In the meantime, what year and model is your truck and engine? And got pics of the engine compartment we can see?
The part number on the choke pull-off (or whatever that vacuum pot is in this case) indicates it's a '78 (or possibly later) model year part. Is that what your truck is?

Paul
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