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Vacuum Modulator and Line Missing?


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Hey everyone! After not posting for about a year, I'm finding myself working on the 620 a lot again recently, and as such, finding all kinds of new questions to ask. A buddy of mine who knows a lot more about vehicles than I do pointed out that my truck appears to be completely missing the vacuum modulator and vacuum line. I'm curious if there is any reason why a previous owner would have removed these parts, and how I was able to drive the truck home without it.


I have the '73's 3-speed Auto Jatco 3N17B transmission with an L16. When I drove it home, it wasn't getting much power while climbing a hill and I later attributed that to the vehicle having no fuel and me not knowing it, but I'm guessing it could have also been that the carb wasn't tuned, that the missing vacuum modulator and line prevented it from downshifting, or possibly a combo of all 3 issues.


Here is a quick snapshot of where it should be:




So is this something I need to order and throw back on there, and is the fact that it is missing sensible to anyone?



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There (should be) a rubber elbow at the modulator, a hard line up to the intake by the vacuum fitting for the brake booster and another rubber hose to this fitting... which begs the question is this a vacuum leak right now????


The intake vacuum to the modulator tells the automatic how much load is on the engine. Low vacuum delays the up shift, higher vacuum allows an up shift if the governor allows it.




Kick down is by a switch on the gas pedal connected to the round solenoid to the right of the modulator.



Why is it missing. Maybe the automatic was replaced and it got lost????

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Thanks datzenmike. I'll be ordering a line and modulator in the next few days. It has definitely been interesting seeing what goes missing on these older trucks, but I'm enjoying learning more about them from the community here! 

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Found the part number(s) for the Vacuum Modulator line on carpartsmanual, but can't find any hits online to order a replacement part. Plenty of hits for other 60's/70's vehicles, but not for Nissan/Datsun.



Is this something I could just make out of brake line if I bend it correctly and connect it to the tube elbow? Doesn't look like there is enough demand for this part for it to be sold in the correct length and shape.


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 Take the modulator valve out. Be careful there is a small pin inside, don't loose that.



Take the modulator with you to an auto parts store and find 6" of rubber hose that fits the modulator valve. Buy 3' of brake line or copper tubing of a diameter that fits the hose snugly and bend your own.




Congrats, you fixed your car for under $5. Welcome to vintage car ownership. Original parts are not always available and sometimes you have to make your own.

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Picked up the parts - rubber hose, steel line, and will be borrowing a line bender from a friend to make it tuck in nicely. Now I'm just having trouble with where on the intake it is actually supposed to route to. From the description and image above: ..."a hard line up to the intake by the vacuum fitting for the brake booster and another rubber hose to this fitting". I can see in datzenmike's reference pic that the modulator line ties in with the main vac line at the intake port closest to the driver's side. Mine already has a hose there that wraps all the way around to the vacuum assy-control on the distributor:




Is this set up incorrectly? There are two additional ports on the intake that were entirely plugged up with some sort of debris or filler. I'm not sure if this was just settled dirt or if someone purposely packed it with filler in an attempt to block the ports, but it took some work to clear them out. Are one of these ports where the distributor vacuum hose should route, opening up the connector at the booster hose joint for the modulator line?


I also found this guy hanging below the booster with no hoses connected to it, and the hose snapped right at the frame as soon as I moved it:



Followed the line back underneath the truck and it led under the rear passenger fender above the fuel port. CarPartsManual labels this part as the "Flow Guide" and it appears to route to the air cleaner and carburetor, possibly more. Guessing most of this was yanked out when two owners back put a 32/36 carb in (an EMPI EPC), and it never got set back up with the most recent PO took that carb off in favor of a stock-style carb. I figured this flow guide and its lines don't have anything to do with the modulator line, but it does have to do with vacuum which is a total mess on my truck as a whole and I'm guessing its important to have connected. 





Any chance for a reference pic of someone's properly connected distributor line, as I'm guessing its improperly routed and that's why I'm having trouble figuring out where to connect the modulator line. And for that flow guide, is this part of the "optional" emissions components a lot of people delete, or is it actually crucial to having a well-running truck? My guess is that a lot of these reroutes and strange missing pieces and connections are all due to an attempted emissions delete and were set up for the EMPI carb. I have no preference as to which carb I use out of the two I have (EMPI EPC 32/36 - found video proof of the truck running with this carb on it in 2016, or a $90 ebay carb in the Hitachi style that the PO ordered and put on for some reason), so if it would make more sense to put the EMPI back on and only connect what is absolutely necessary, I'm all for that. And if its any help, I believe this is a 49 state truck, not a Cali.

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This hose from the distributor to the intake is totally wrong. It should go to the valve cover side of the carburetor. You'll have to ask someone which one as I don't do Webers.


The short hose from the thermostat to the pipe going down in front of the engine looks kinked. In addition there should be a Y there and another hose to the intake at the base of the carburetor. Can you take a picture of this area?






This is the flow guide valve and unless you have all the pieces, I would just set it aside for now.

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Here is the thermostat hose and line to the intake:


bends back rather sharp but hose doesn't pinch



Edited by FishyLemons
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That looks ok as long as that hose isn't kinked. You may be able to bend the hard line down slightly to open up the bend.


I see the screw is out of the slot on the distributor. Probably because running intake vacuum, you will probably need to advance once the vacuum advance is set right.

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