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Need Carburetor expert. Fuel coming out of vacuum

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spddm0n View Drop Down

Joined: 12 Jun 2019
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    Posted: 12 Jun 2019 at 5:01pm
Hi all - 

I'm new to the community!  

I just picked up an '85 720 King Cab 4X4!  I've been reworking a number of things on the truck and it's starting to look pretty good!

Does anyone know what these two ports on the driver-side of the carburetor are for?  I recently bought an '85 720 KC 4X4 that had the wrong carburetor on it.  It didn't run well.  I picked up the correct carburetor at a salvage yard and a rebuild kit from Napa.  I rebuilt it, along with having the air/fuel ratio solenoid rebuilt by a professional shop.

After installation, I realized I have two ports on the driver-side that aren't connected to anything. I believe they may go to the (high) altitude compensation system, (solenoid mounted on the passenger-side inner fender apron).  I believe I have confirmed the device on the fender apron is in fact just that - the altitude solenoid (for equipped vehicles).

Most of the pics of this carburetor that I can find online, show these two ports capped off.  ??  I found an online reference to an unknown Nissan 720 Service Manual, suggesting these two caps are removed when installing the altitude compensation system.  Pic below.

When I run the engine without anything connected to these two ports, the engine doesn't run well due to a vacuum leak from the lower of the two ports.  When I cap it off, it runs better until the engine warms up and idles down.  Then it stumbles, hesitates and dies.  If I remove the cap, fuel comes out of that port! :)  WTF?!   The upper of the two ports doesn't seem to affect how the engine runs.

I tried running vacuum lines from these ports to the altitude solenoid, but the same issue occurs.  Same problem...the line to the lower port fills up with fuel.

I don't have any vacuum diagrams of the altitude system to suggest which lines go where.  When connecting the vacuum lines up the other way causes the engine to run badly.  

I can't imagine fuel is supposed to be in that line by design.  The hoses appeared to be vacuum hoses, not suitable for fuel.

I removed the carburetor and checked/adjusted the fuel bowl float level.  I believe it is correct.

To determine where the lower port leads to inside the car, I put compress air into it.  It escapes through: 
1 - The jet in the bowl, sourcing fuel to the secondary barrel on the carb.
2 - The nozzle into the venturi in the secondary barrel.
3 - There is also a small hole on the inside of the barrel, toward the top,that also seems to allow some of this compressed air out of it.

Has anyone any expert knowledge of the internal intricacies of this carburetor?  Or, the correct routing of the vacuum lines to these two ports?  Or, experience with the altitude compensation system?

1985 Nissan 720 King Cab 4X4
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520KingCab View Drop Down

Joined: 23 Mar 2014
Location: Surrey BC
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 520KingCab Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jul 2019 at 10:23pm
Hi there, are you still looking for information? I do have a manual showing it. Thanks, Wayne
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MOCMAN View Drop Down
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Joined: 28 Nov 2010
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MOCMAN Quote  Post ReplyReply Direct Link To This Post Posted: 19 Jan 2020 at 1:17pm
I would just replace it with a Weber carb kit made for your truck. You will be Soooo much happier. Better performance and reliability by a long shot.
John 3:16
"There's a fine line between "Hobby" and "Mental Illness"
1985 Nissan 720 KingCab 2wd 5spd.
1995 Nissan Hardbody 2wd Reg. Cab
2014 VW passat SEL VR6
1966 VW Beetle
2005 Ford Explorer XLT 4X4
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