vacuum line(s)

[70MTRoadster]

OK, so am about to transplant the U20 into the my 70 1600. My question is do I run vacuum from the front carb over the top of the valve cover to the distributor vacuum advance? Does anyone use the funky what'sit on the thermostat housing with its 3 barbs? I'm guessing it was for smog stuff. I've owned 2 each 2000's and worked on them all the time back in the 70's but 50 years has dulled my memory and about everything else too.

[jrusso07]

Yes on the vacuum line routing - one of my cars has a clip that goes under rear cam cover nut that the line routes through...not sure if it is stock but it keeps it looking tidy. I don't use the funky thingy on the t-housing. Not sure what it is...I assume some type of temp sensor that opens a vacuum line to some emission thingy that I don't have...maybe the air pump system. I have replaced them on all my 2000s with pipe thread cap (square drive cap) because I thought the funky thing looked too funky for my style.

I added a pic of the t-housing with the cap installed (not my car)

[MattC]

And another reference...here is mine with the vacuum line over top through a small clip on the valve cover nut.

2497CEC7-964E-4059-9DB0-EE9169050955.jpeg

[70MTRoadster]

Thanks! Always nice to get an answer quickly here. I will pull the funky part and plug as you have done.

[spl310]

The funky part helps keep the car from overheating

[jrusso07]

The funky part helps keep the car from overheating

Tell us more...?

[spl310]

It has been a long time since I read about it. Basically it retards the timing if the engine starts to overheat, iirc

[david premo]

That would be backwards. The thermo switching valve advances the time to cool the engine. So it has three ports D C & M, distributor, carburetor, and manifold. At idle with the timing retarded combustion temperatures rise and more complete combustion occurs. A fan clutch and thermo vacuum switch are used to prevent the engine from overheating. As temperatures reach about 210 degrees Fahrenheit two things occur, the fan clutch engages and and the thermo vacuum switch changes from carburetor vacuum to manifold vacuum which are inverse to each other and advances the timing. As the fan kicks in and the vacuum switch’s it quickly cools the engine back to about 185 degrees and reverts to carburetor vacuum. This is why engines with the smog distributor run so hot when you get rid of the thermo valve. Basically a smog distributor without the thermo switching valve is a grenade with the pin pulled waiting to explode.
Dave

[spl310]

Thanks. As I said, it's been a while. Since I only have nonsmog cars, I don't have it

[jrusso07]

Thank you Dave, I’m glad I am running non-smog distributors!

[Gregs672000]

Best explanation I've seen. I'm with Joe, glad I don't have to worry about it. As I understand it, Nissan went to great lengths to try to keep the U20 emissions compliant with this system.
I know people have complained about how much emission controls killed power, and in the beginning they did kill off many engines. It's truly amazing how much power modern engines can make while still being so much cleaner than even 10 years ago. Hats off to all those engineers out there who attacked this problem.