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jkelm24

1974 620 Weber DGV & Fuel Evap Line

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Has anyone found a good solution for the two fuel tank evaporation vacuum lines when using a Weber DGV (fuel evaporation ports located on inner left fender below brake master)?  

 

My truck has an l20b swap, but retained the '74 manifolds.  The intake manifold has the EGR removed and the EGR mount capped, and has PCV port and single vacuum port.

 

The intake single vacuum port has too much vacuum for the fuel evaporation lines.

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EVAP...Hmmmm. You could simply delete them or cap them off.

 

I've never taken the time to investigate EVAP when talking about a Weber DGV. Where do they connect on the stock setup?

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Do not cap the tank vent or it will collapse as the level goes down. If you do cap switch to a vented gas cap.

 

Best is to get and install a charcoal canister for this. Tank vent goes to the canister where fumes are stored. Vacuum advance line goes to the purge fitting on the canister and tells it to open when engine running and connects to a vacuum line to the intake. Stored fume reverse through the charcoal and are drawn into the intake and burned. (some also store fumes from the carburetor like below...

 

file.php?1,file=79324,filename=14264_155

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Helpful, thank you.  So I assume the described solution would replace the existing Flow Guide valve?

 

The stock diagram indicates that the lower flow guide valve port should be connected to the crankcase, and the upper flow guide port should be connected to the air filter.

 

Fuel Tank Vapor System

 

http://datsun1200.com/modules/myalbum/photo.php?lid=7798

 

I do not see a way to connect the lower flow guide valve port to the crankcase or PCV line on the L20B.  Does this diagram represent only the L16 and L18?  I think I've seen small 1/4" port outlets on the L16 crankcase breather tubes.

 

Is the charcoal canister the only solution for a Weber DGV and L20B combo without having to vent to atmosphere?  Does the purge line connection to the vacuum advance line have any negative impact on the distributor advance performance?

 

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I assumed the '74 had gotten rid of the flow guide valve. If you have it I guess run with it. It stores gas fumes inside the engine. When started the guide closes the tank vent to the engine and opens it to filtered air. It uses the slight suction from the PCV to draw stored fumes from the crankcase.  You just need to make a connection to the crankcase breather tube that goes to the PCV valve. The L16/18 crankcase breather comes straight out of the block and has a small tube on it for the flow guide valve hose.

 

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L20Bs do not.

 

 

I don't care much for it as fumes are stored in the crankcase rather than a charcoal canister. But it works.

 

The purge and the vacuum advance are closed systems so vacuum remains what ever is sent from the carburetor port. Naturally there are extra hoses and clamps that need to be on and tight so there is no leak.

 

I wouldn't just let the tank vent pipe simply vent to the atmosphere* it's either smelly, might leak fuel in a roll over possibly a fire hazard. I would close it off and get a vented gas cap that will do it's job. 

 

*I would get one of these two systems working properly

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Yes, my 1974 has the flow guide valve.  But it's also had an L20B engine swap so the additional crankcase port is not available.

 

Looks like I'll have to get a charcoal canister or vent to atmosphere.  

 

Thanks for the feedback!

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Good timing...I was just thinking about how I'm going to handle this with dual Mikuni 44s. 

 

I like the concept of retaining the EVAP canister, and doing as mentioned. If I were to abandon it, I'd probably have a 1-2 pound pressure release check valve as a fuel tank breather to allow air in, but hold most of the fuel vapors in the tank (sealed fuel cap). The check valve is similar to the Suzuki Samurai, except when the pressure releases from the check valve, it goes to the EVAP canister.

Though I like the EVAP canister, but it messes with the simple look I wanted in the engine compartment (vanity). But I also hate the smell of fuel vapors in my garage on a hot summer day.  

 

I haven't researched it yet, but how does that crankcase vent pipe come off of the L20B block? Sorry...if it's plain obvious, but I haven't really looked at it yet after removing the stock exhaust manifold. . 

Edited by 2wheel-lee
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L20B doesn't have a crankcase port available for the fuel evaporation line.  It only has the PCV breather port.

 

Two options:

 

1)  Cap the fuel vapor line in the engine bay and use a vented gas cap.

 

2)  Run a charcoal canister.  The fuel evaporation line will run into the canister.  The purge valve draw ports will then route to the intake (one will 'T' into the distributor vacuum advance line and the other will draw from an available port in the intake manifold). 

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Good timing x2 because I was wanting to get rid of those lines and have no place to run them on the KA24DE.

 

,

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46 minutes ago, jkelm24 said:

L20B doesn't have a crankcase port available for the fuel evaporation line.  It only has the PCV breather port.

 

Two options:

 

1)  Cap the fuel vapor line in the engine bay and use a vented gas cap.

 

2)  Run a charcoal canister.  The fuel evaporation line will run into the canister.  The purge valve draw ports will then route to the intake (one will 'T' into the distributor vacuum advance line and the other will draw from an available port in the intake manifold). 

 

2/ is the better. It''s set and forget.

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9 hours ago, datzenmike said:

 

2/ is the better. It''s set and forget.

 

I agree with 2-wheelee, I dont like the look, but understand it is more better because if contains the smell that i am not a fan of either.

 

Where would I get one? I was hoping that someone makes a clean looking canister, but I cant locate one.

 

https://www.summitracing.com/parts/rnb-911-480/overview/

 

 

EDIT:

check this out - has p0tential??

 

https://www.usplastic.com/catalog/item.aspx?itemid=104120&catid=578

 

.

 

 

 

Edited by FrankRizzo
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Setting up an EVAP system can be a bit complex. The one linked to at Summit appears to have an electrical connection. It says that this canister is for certain Infiniti models. I don't know their EVAP system, but I'm guessing that connection is either a Fuel Tank Pressure (FTP) sensor (also assuming the canister is located next to the tank), or it could be a canister vent shut valve - a completely different level of complication from the simple 620 system.

 

I believe the diagram Mike posted near the top of this page is similar to what was on my '75 620. However, the one port that goes to the carburetor fuel bowl vent was non-existent on mine. The canister appeared that it may have a provision for it, but it has a factory-type plug on it, so I have doubts that it was plugged during the life of the vehicle. The smog pump was removed at one point, so I don't know for sure. 

 

The advantage of the common 620 canister assembly is just that: it's simple. No electric vent valves, purge valves, FTP sensors, etc. It's merely activated by vacuum. I just don't know the timing or specific operation (e.g., how much vacuum to open valve or even if the valve is variable). If this were a modern Honda, I could describe how the system works in great detail (I did, in fact, in a previous life). The downside of the simplicity of it is that it effects the fuel mixture in the engine differently at different times, and there's no feedback loop to compensate for it, such as adjusting the fuel trim on an EFI system when the O2 sensor detects a rich mixture.  

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California models may have had the carburetor fuel bowl vent. I think most '80s carburetors did and by 86 everything was EFI anyway. Depending on the wrecking yards an old Chev unit will do or anything else really.

 

The purge signal is taken from the vacuum advance line. All it does is open a valve to to a small orifice, to let intake vacuum suck the fumes out into the intake to be burned. The amount is extremely small and on start up the choke is on anyway. If a warm start up there wouldn't be time for fumes to be collected.

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Though once the purge valve is open, vapors are also drawn from the fuel tank via the line between the canister and tank. Perhaps also minuscule, it's still there. The amount of vapors in the tank are also affected by how much fuel is in the tank, driving conditions, and even road temperature (affects tank temperature). Again, likely minuscule, if the orifice you mention is small. Do you know how small? 

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Small enough to not affect the intake vacuum. Being on the ported vacuum advance, there is no purge signal at idle anyway and only when driving.

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I think my existing line(s) is/are just sitting under the brake booster...so if I dont small gas, maybe i just leave it alone? Or add that inline carbon canister I linked to above.

 

A lot of this (great) info does not seem to apply to the KA24De...like float bowl vents...but its a great start. At least I know how the shit is supposed to work now!

Edited by FrankRizzo

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Duralast vapor canister part #VC4180 is listed as a direct canister replacement for the Datsun 720, and might be a good option for the 620.  Although it's a bit spendy (Autozone).

 

Also see Standard Motor Products #CP3051 and WVE/Airtex/Wells #4B1257.  You can find these listed on RockAuto under 720 model years 1981 and up. 

 

But these all would be associated with the Z motors.  I'm not sure if the vacuum draw from the L motors would operate the valve properly, and would look to some others for help on this one.  I can't imagine the vacuum draw would be much different between the L and Z motors.

  

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I forgot that you're using a KA24 with EFI. I don't know that engine or how its emissions system works, but I'd bet that it has an electric purge valve that's commanded open by the ECU. You might also check to see if the original vehicle that your engine/ECU came from used an EVAP canister shut valve or fuel tank pressure sensor (terms may be slightly different). If you can research that a bit, you might find that all that EFI emissions stuff is so much easier to work with (especially if it's OBD2). For example, if you can get all those OBD systems operating correctly, you can utilize DTC troubleshooting more effectively. Yes, I like working with OBD2 - and the later the better, as the codes become more and more refined. 

 

Sadly, I've forgotten everything I once knew about carbureted systems (in automobiles). 

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X2 on the EFI. It is so much simpler than carbureted systems. Once you get comfortable with it, that is.

 

These LS Erod swaps I have been doing have far fewer components than the original carbureted engines that I take out.

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Thanks for that ^. its from a 1993, so no OBD. I have to pull the ECU and "count the flashes" for trouble codes...it seems so archaic, but it works.

I dont have any issues right now other than a couple of unhooked hoses tucked away under the hood.

 

thanks again.

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Got a stupid question....

I am putting a weber 32/36 dgev in my 74 620 and I have the manifold with the EGR. Going to take out the EGR control valve and solenoid and use a block off plate there. On the flow valve can I cap the line that recirculates through the air breather and use a vented gas cap? I actually saw a picture on here where somebody had a vacuum line from that valve going to the 90 degree fitting on their weber air filter. I want to keep the line going into the crankcase by the pcv. I don't want to mess with a charcoal canister. Trying to keep ir simple.

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You're keeping the PCV valve right? Always keep the PCV working. Technically an emissions device but the emissions that it prevents being expelled into the environment are what collect in and dilute the crankcase oil making the engine insides filthy. A working PCV extends the oil change interval and easily doubles the life of your engine. They started being used around '62. In the '50s you would be very very lucky to get 100k out of any engine.

 

Yes you could just block the line back to the gas tank and put a vented cap on. If capping this line you don't need the hose to the block vent tube. Only reason for the hose to the air filter is to filter the air returning to the gas tank as the level goes down. I believe the vented gas cap only lets air in. On hot days the gas and air inside the tank will expand and can't get out through the flow guide valve like they should. Something to keep in mind. Simpler yet to just leave the flow guide valve alone.

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Yes Mike, I am keeping the pcv valve. Correct me if I am wrong, but from what you are saying, I can block off both ports coming from the flow valve and run a vented gas cap? I was thinking I had to keep the bottom line to the crankcase and just cap the top line which returns through the air cleaner. Screenshot_20200701-155238.thumb.png.be18b418dbb7e2ca10670c6d31155fd2.png

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On a side note Mike, since I have had the truck the lines on the stock air cleaner were never hooked up. When I removed the stock carb, that return line to the flow valve was "T'd" with one line going to a port on the EGR manifold and one going to a port on the BCCD. Never could find any diagrams of that in the FSM. Thanks for your help....attached a picture, just disregard my labeling.562139851_20200624_154027(2).thumb.jpg.d57709ca331aa9ff0bb13871a97b389a.jpg

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One more question Mike (sorry..lol)

When I remove that EGR control valve and solenoid to block off do I just cut that wire to the control valve. I dont see a plug connection, looks like a old butt connector. Thanks

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