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Vacuum at carb port
https://slantsix.org/forum/viewtopic.php?t=12044		 Page 1 of 2
Author:  dvdart [ Wed Mar 02, 2005 4:14 pm ]
Post subject:  Vacuum at carb port
I have a vacuum of about 10 in Hg at the carb port that goes to the distributor advance. It is a Holley 1920. Should there be a vacuum here? It is the only port I have on this carb besides the one for the pcv.
Thanks,
Dave
Author:  Rust collector [ Wed Mar 02, 2005 4:19 pm ]
Post subject: 
It should not have vacuum at idle!
Author:  dvdart [ Wed Mar 02, 2005 4:30 pm ]
Post subject: 
What could cause this?
Author:  Dennis Weaver [ Wed Mar 02, 2005 4:42 pm ]
Post subject: 
Quote:
What could cause this?
Possibly the curb idle (throttle butterfly opening) set too high (too far open).

D/W
Author:  SlantSixDan [ Wed Mar 02, 2005 6:26 pm ]
Post subject: 
Quote:
It should not have vacuum at idle!
Not necessarily true. If you look in the factory service manual or parts book for just about any year slant-6 vehicle, you'll find one carburetor used for manual-transmission applications, and another for automatic-transmission applications. Look closer, and you'll see that all the specs seem identical. Same jetting, same venturi size, etc. etc. So what's the difference? One has the vacuum spark advance port located below the throttle plate at idle, the other has it above. This is documented in a 1978 Chrysler Corp. publication on driveability and emission control—my copy is currently archived in Denver, so I can't recall which transmission gets which kind of carburetor, but the idea is that some carburetors are specifically designed to supply at least some distributor vacuum even when the idle is set correctly.
Author:  Dennis Weaver [ Wed Mar 02, 2005 7:58 pm ]
Post subject: 
Quote:
One has the vacuum spark advance port located below the throttle plate at idle, the other has it above. This is documented in a 1978 Chrysler Corp. publication on driveability and emission control...
Hmmmmm, that's interesting.

D/W
Author:  Rust collector [ Thu Mar 03, 2005 1:52 am ]
Post subject: 
Didnt know that...
Author:  Slant6Ram [ Thu Mar 03, 2005 6:38 am ]
Post subject:  There is always an exception.
Quote:
This is documented in a 1978 Chrysler Corp. publication on driveability and emission control
Duely noted, :shock: but remember that we are talking about a holley 1920, and I don't recall any being used on 1978 models.

The reason give by others -should- apply here. Check the idle mixture adjustment and then try to reduce the idle speed. Otherwise you could pull the carb and spray carb cleaner through the port, to see if it exits into the carb above or below the throttle plate. (with the idle screw backed out and the throttle completely closed)

A common reason for the idle screw to be set outside of normal operating peramaters is because the PCV valve is clogged or not connected. The air flow from the PCV is critical to getting a good idle setting with a 1920.
Author:  SlantSixDan [ Thu Mar 03, 2005 7:32 am ]
Post subject:  Re: There is always an exception.
Quote:
Quote:
This is documented in a 1978 Chrysler Corp. publication on driveability and emission control
Duely noted, :shock: but remember that we are talking about a holley 1920, and I don't recall any being used on 1978 models.
Please re-read what I wrote&mdashthe part about "ANY YEAR parts catalogue or service manual". This info applies clear back into the non-emission-control era. Having carefully dissected a great many slant-6 and other carbs from many different years over the years, I can tell you that there are pre-emission slant-6 carbs with the advance vacuum port above the throttle plate at idle, and there are pre-emission slant-6 carbs with the advance port below the throttle plate at idle.
Author:  dvdart [ Thu Mar 03, 2005 8:02 am ]
Post subject: 
Thanks for all the responses.
As soon as I can find a carb mounting gasket I will take the carb off and make sure where the port is entering, above or below throttle plate.
I don't know how to post photos here so here is a link to photos of my carb.

http://photos.yahoo.com/dvdart1

The second photo shows the vacuum port I am using at the base of the carb. To the left, or rear, of the mounting bolt.

It is a Holley 1920 and here are other numbers listed on the carb:
3398 1S 0151
3419582
Maybe it is the wrong carb for this car?
1965 Dodge Dart 225 engine.
Thanks,
Dave
Author:  Dennis Weaver [ Thu Mar 03, 2005 8:20 am ]
Post subject: 
Yep, that's the right "tap" for the distributor vac. Question: Where is the "tap" for the PCV on this carb... It's not evident to me in the pics. I've seen them on the drivers or the passenger's side. If yours is not hooked up (or non-existant) that could very well be your problem.

D/W

EDIT: Never mind, I see something now... Is that hose on the driver's side going to a functioning PCV valve?
Author:  Slant6Ram [ Thu Mar 03, 2005 8:29 am ]
Post subject:  Make it work
Them #'s don't mean much to me, but I'm not big into #'s

You should be able to run that carb on any slant six I ever saw. A 1920 for sure.

Yes, the port that you've described is the correct one to hook to the vacuum advance, and No, there shouldn't be any vacuum at idle.

I can't see if the other hose is going to the PVC. Is It? Have you replaced the PCV valve recently? This carb is designed to have a PCV and won't work properly without one. (ask me how I know)

What is the adjustment on the idle mixture screw? Hint, count turns till it bottoms out in the hole (don't tighten, just till it stops turning), then return to origional adjustment.
Author:  SlantSixDan [ Thu Mar 03, 2005 7:12 pm ]
Post subject: 
Quote:
No, there shouldn't be any vacuum at idle.
Slant6Ram, you're just not necessarily correct on this. One more time: depending whether the spark port is above or below the throttle plate by design on this carburetor, vacuum at idle may or may not be present.
Author:  SlantSixDan [ Thu Mar 03, 2005 7:16 pm ]
Post subject: 
DVDart: That carb's perfectly applicable to your car, but please rework the fuel line that goes from the fuel pump to the carburetor, ditch the plastic fuel filter and install a metal one in a different location. Your present setup is not very safe and at best will cause driveability problems.

Do a forum search on "30R9" to find the setup.
Author:  argentina-slantsixer [ Thu Mar 03, 2005 7:19 pm ]
Post subject: 
THE 1922 we had down here for auto trans has the vac port spreading below and above the throttle plate. It's like a 1mm wide rectangle perpendicular to throttle plate. the portion that extends below the throttle plate is like 1 square mm. So, I HAVE idle vac advancing on a 1922 (wich I already posted to be an argentinian improvement of the 1920)
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