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1978 620 Vacuum line help!

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So, recently got this 1978 Datsun 620 and bought it without a smog certificate, so, now I'm trying to get it smog ready. It's a clean car/engine...been well maintained but the carburetor/vacuum lines have been modified. I've scoured the internet looking for vacuum line diagrams and none are detailed enough to answer my questions. Seen a few posts on here about vacuum lines, but again, nothing that really helps. I'm new to Datsuns and kinda green when it comes to working on cars but, this truck seems awesome to learn on and I'm excited to get it running well.

If anyone can provide photos of their vacuum system or vacuum system diagram chart, I'd be ever grateful. Need to get this baby smogged and legal so I can drive it! Thank you!

(I have detailed pictures of the vacuum configuration currently setup of anyone is interested?)

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Got most of the lines connected. Still wondering about a few things...




What is that little nozzle on the carb and where is that open vacuum line supposed to go?


Also wondering what this is?




And then theres this




Wondering where these lines are supposed to go aswell.

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The '78 L20B has two ported vacuum lines on the carb base closest to the valve cover. Neither have a vacuum signal at idle but increasingly do as the throttle is opened. Likewise there is virtually no vacuum signal at wide open throttle either.


The one most to the front goes to a hard line and from it to another hose going to the distributor vacuum advance (this must be kept) and through a T to another hose to the charcoal canister purge nipple. Thus when the engine is running but above idle, vacuum is directed to the distributor and the canister purge valve.


The rear most port is for the EGR. It goes to a Thermal Vacuum Valve mounted in the thermostat housing that only opens above a specified temperature. This means that there can be no EGR at idle or if the engine is cold. From the T V V to the Back Pressure Transducer and the EGR valve. Between the BTV and the EGR, exhaust gasses are recirculated (subject to above idle, warm engine) and varied proportionally to engine back pressure (load). Strictly speaking this has no effect on full throttle running and disabling it doesn't do much. Keeping it, makes is run 'cleaner'.

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Yes! Finally got a decent picture up!

So...I'm trying to figure out where the two nozzles (one off the carb, one is a hard line) on the lower part go to.

Also, what is that little unit at the top of the photo?


OK, one more...where are these two going?


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The '78 L20B has two ported vacuum lines on the carb base closest to the valve cover.

I'm only seeing one near the valve cover (above picture) near that open hard line.

Thanks for the response by the way...that info should help determine if I have the right connections elsewhere.

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This is an automatic????


The long hose with the ? is wrong and should go to the carb. There should be TWO fittings on the carb. It should go to the front one. It looks like you are running on intake vacuum for your advance. It should be ported vacuum from the carb.


The round red circle should go to the second port on the carb. The rear most one. This would be for the EGR. It will run fine without this.

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Yes, it's an automatic.

So here's the closest diagram I've found.



There's 4 hardlines. Two look to be fuel running to the carb connection in below pic.


The charcoal canister has three ports


1. Fuel tank (which is where it's going)

2. Purge (which is connected to manifold intake port through a t which then goes to the AB port)



3. Dist VC (which runs to a hard line with a t, one end going to the distributor the other is the long house that's not supposed to be there.



This is the only vacuum port I see on the carb (which was plugged off)...



Only other port is this (which is connected to two hardlines running round the front of the engine (fuel lines?))



Sorry for such the headache! Hope that's not too complicated. Really appreciate any help/input I can get. Have made appointment with mechanic to have this sorted out...but I'd love to be able to do it myself (with ratsuns specialists help of course!)

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I agree. This is one of those things that you would be better off doing yourself. It may take a bit of effort, but then once you have the knowledge of what does what, it will make future repairs easier because you will already have a basic understanding of how things work.


Sometimes I tell people that their time would be better spent by having a mechanic do it, but not this time.

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Unless he's older or a datsun guru, you'll do just as good if not better than your mechanic....

Most mechanics now a days get lost if they can't plug it into a computer...

He's just gonna have to come here and ask datzenmike what to do anyhow..

Haha! Yeah, I hear that (datzenmike knows his shit!)

He is older but he looked at it when I took it in for smog and just said it'd be time consuming $$ (putting in the time that I'm doing right now to figure it out.)

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This is totally the wrong carb for this engine. If there is only one ported vacuum outlet it must be for the vacuum advance. This carb is from before '74 when there was no EGR. It also has a manual choke which were done away with in '72?. There is no idle cut solenoid. I would say a '71 521 or maybe from a 510 but either had an L16 engine. The fuel line to the carb comes up from the side.. all L20Bs the banjo fitting turns towards the valve cover.

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