Where does this hose go? Weber 38/38 Install with Stock Intake.

[Rich Elliott]

Hi All,

 

I read some other threads and I am still confused on one specific hose.  I'm installing my Weber 38/38 carb on my '76 2002 in preparation for an engine swap to my '70 2002.  I'm keeping the fuel return valve and using the stock intake manifold (for now).  Here is a pic of the fuel return valve:

 

 

This small hose "A" off the bottom of the fuel return valve was previously connected to the side of the stock Solex that I just removed.

 

This is a pic of my new Weber carb:

 

 

Here you can see the brass nipple on the front of the Weber carb.   So my question is, should I connect A to B? Or is this brass nipple for something different?

 

Thanks,

 

Rich Elliott

 

 

 

 

 

[Mike Self]

The nipple labeled "B" on your Weber is a vacuum port; if you connect that to the fuel return valve, it seems to me that it will suck gas fumes into your carb, possibly messing up the mixture.

 

I removed that whole fuel return setup when I installed my Weber (back around 1981) so never really tried to connect it.  I'm sure others on the FAQ who still have that system intact will chime in.  

 

mike

[Rich Elliott]

Hi Mike, 

 

Thanks for the response!  The more I read, the more it sounds like it will be okay to connect "A" to "B".  In other words, it's okay to connect the vacuum hose from the fuel return valve to the vacuum nipple on the Weber. 

 

According the jmik on this post:

The small hose at the bottom of the fuel return valve is a vacuum hose.  It's a vacuum hose that operates the valve.  Nothing goes through it.

 

Alternatively, it should be okay to connect the vacuum hose from the fuel return valve, "A", to a vacuum nipple on the intake manifold.  I think for now, I'm going to connect A to B as shown in my photos and see how it goes.  

 

Also, I found this page helpful:

http://www.bmwmobiletradition-online.com/bmw/showparts.do?model=2215&mospid=47140&btnr=13_0679&hg=13&fg=10

...as it describes the small hose from the bottom of the fuel return valve (#19) as a "VACUUME HOSE".

 

See #19 below...

 

 

['76mintgrün'02]

If you are using a distributor with vacuum advance, you'd typically plug that into B, on the Weber (ported vacuum).

 

Another option for hose A would be one of the vacuum nipples on the manifold; on either on runner one or four (manifold vacuum). 

 

(I believe John76 has posted information about how the fuel return valve was originally plumbed in).

[N2MY2002]

Im running a 38/38 and don't remember even seeing a hose for that but my different question back at ya is, are you using the stock intake manifold? If so, have you mocked up the weber to see if the butterflies open and close without hitting the manifold? Usually, one uses a Cannon intake or hog out the stocker.

[worzella]

 

My old morning eyes and brain just getting a dose of caffeine, but as I follow this diagram, the fuel return valve vacuum line connects to the carb for vacuum and also provides a splitter to send vacuum to the advance/retard switchpot. So I would interpret this as meaning:

 

o Yes, you can connect your return valve vac hose to the Weber ported nipple, but as Mint indicates, you want vac also still to your advance, hence the splitter.

 

o Probably also could do as MInt says and use carb ported for the advance and connect your return vac to a manifold vac port... but I vaguely know that the Weber ported vac and manifold vac levels are different at different times during idle/accel/backoff, so not entirely sure what the original design intended for the return valve vac. If, as I have read, its point was to circulate gas through the tank when not needed by the carb, then seems you want the return valve OPEN at idle and closed when stomping. Assuming OPEN means letting gas bypass the carb.....Exercise left to the reader  Should be able to disconnect a couple fuel lines and suck on the vac and see how applying vac affects the valve. 

 

Randy 

 

[Rich Elliott]

Awesome guys.  Thanks for the info and help.  My distributor vacuum is currently connected off a vacuum nipple on the stock intake manifold via the little bell things hanging off the firewall.  I don't want to touch this (right now at least).  I'm gonna attack the distributor a little later when I work on smog delete.  I'm trying to just get the 38/38 installed and working right now. 

 

Randy,  I like your idea for testing the fuel return valve.  I'll give this a try.

 

N2MY2002,  I just got done mocking up the throttle linkage that came with my Weber last night and it seemed to be working good. Based on your message, however, I'll have my son operate the gas pedal and watch everything up close to make sure there is no binding and that the butterflies are opening and closing properly.

 

Thanks again!  

Edited March 23, 2018 by Rich Elliott
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[John76]

The fuel return vacuum line should be connected to a manifold vacuum nipple on the intake manifold. This will open the fuel return valve (sending some gas back to the tank) during idle (high vacuum).

If you plug this into the ported vacuum ("B") on the Weber, you will get no vacuum at idle (max gas flow to the carb) and less gas when the vacuum kicks in. Opposite of what you want!

 

This fuel return valve was designed to be connected to the front vacuum port on the Solex carb. This is a "reverse"  ported vacuum source. It gives vacuum at idle (sending fuel back to the tank) and no vacuum at off idle (producing max gas flow to the carb).

 

Since there is no "reverse ported" vacuum spigot on the Weber, connect your line "A" to a T-fitting on the vacuum line going to the air pump control valve.

 

John

[worzella]

John76 - Man I love it when the real explanation is provided. Very nice!!

[Rich Elliott]

19 hours ago, John76 said:

The fuel return vacuum line should be connected to a manifold vacuum nipple on the intake manifold. This will open the fuel return valve (sending some gas back to the tank) during idle (high vacuum).

If you plug this into the ported vacuum ("B") on the Weber, you will get no vacuum at idle (max gas flow to the carb) and less gas when the vacuum kicks in. Opposite of what you want!

 

This fuel return valve was designed to be connected to the front vacuum port on the Solex carb. This is a "reverse"  ported vacuum source. It gives vacuum at idle (sending fuel back to the tank) and no vacuum at off idle (producing max gas flow to the carb).

 

Since there is no "reverse ported" vacuum spigot on the Weber, connect your line "A" to a T-fitting on the vacuum line going to the air pump control valve.

 

John

 

Hi John!

 

Perfect.  This is exactly what I needed.  Thank you so much!

 

So, based on your expert advise, I have setup a T-fitting to connect the vacuum line from the fuel return valve to the the vacuum line going to the air pump control valve:

 

In addition, I plugged up the vacuum nipple on the Weber 38/38:

 

Thanks again!

 

Rich

 

 

 

 

 

[worzella]

The devil on your shoulde is going to start whispering “De-smog me!”

 

Which makes sense since I suspect Satan would want extra pollution. 

 

??☠️

[Rich Elliott]

On 3/25/2018 at 1:16 PM, worzella said:

The devil on your shoulde is going to start whispering “De-smog me!”

 

Which makes sense since I suspect Satan would want extra pollution. 

 

??☠️

LOL!  Yes.  That is definitely on the list.  Especially before the engine / transmission swap out to my '70 2002. 

 

The next items on my list for me right now are:

• Adjust valves (hopefully have it done today >> first time ever on a BMW.)
• Paint valve cover and re-install with new valve cover gasket.
• More engine and engine bay cleaning.
• Get my new oil breather catch tank installed (Got some oily blow by and I want to keep the new carb as clean as I can).
  https://www.ebay.com/itm/Billet-Aluminum-Baffled-Engine-Oil-Breather-Catch-Tank-Can-Reservoir-Black/192284183166?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
• Change the oil.
• Flush the radiator fluid. 
• Adjust timing to 25 degrees at 2,100 rpm.
• Play with carb settings: idle volume and warm idle settings.  It idles at about 500 rpm when warm, I would like it a little higher.
• Oh yeah.... I still need to get the title transferred to my name...

(Recently completed new distributor cap, rotor, wires, plugs, and installed Weber 38/38.)

 

It's running so much better with the new Weber 38/38 compared to the 1,000,000 year old Solex that was there before.  It's a night and day difference!

Edited March 27, 2018 by Rich Elliott
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[dbmw2002]

CD would recommend making a pool toy out of the return valve mechanical, still room in the neighbors pool. As long as your fuel pump is pumping proper pressure you don't need it. The less vacuum you use will make the engine run stronger. ?

[Rich Elliott]

1 hour ago, dbmw2002 said:

CD would recommend making a pool toy out of the return valve mechanical, still room in the neighbors pool. As long as your fuel pump is pumping proper pressure you don't need it. The less vacuum you use will make the engine run stronger. ?

 

Okay, thanks Dave.  I'll keep that in mind.  Getting rid of the return line would probably make my engine swap to the 1970 '02 easier anyways.  I'm not even sure if my 1970 has a return line. I better check...

[jimk]

9 hours ago, dbmw2002 said:

The less vacuum you use will make the engine run stronger.

You are confused between vacuum pressure and air flow.