17 hours ago, jpc3006 said:

Dadsun,

FYI, when I replaced the flat-tops on my '73 with round-tops from a '71 many, many years ago I bought a new thermostat housing and the metal coolant tube that runs around the back of the head from the intake manifold for a 1971. This eliminates the part you call the "sensor" altogether and routes the coolant from the thermostat housing through the intake manifolds and back to the water pump.  Using the 1971 parts also eliminated the need to plug the extra connections as there is no provision for coolant being supplied to these carbs. I have run the car this way for over 35 years and have had no problems; so my answer to your 

"QUESTION: Can I just get rid of this 5/8" hose and sensor altogether for round tops? That would only leave the coolant flowing through the intake manifold on that side of the engine compartment." 

is yes, if you are installing 1971 round-tops as that is the stock configuration. 

This is exactly what I'm in the process of doing.  I came across an old thread where @siteunseen had the very coolant hardline I needed if I wanted to avoid having to plug a bunch of unwanted ports.  I was also able to take the old manifold coolant connector off of my flat top setup and screw it into the new thermostat housing I ordered.  It was a much smaller threaded opening than in my old thermostat. Pic:

 

All I think I need now is the coolant hose reducer segment to go from 3/4" to 1/2" on the passenger side of the engine. Checking my local industrial parts store. Pic:

I'm glad to see you have that "Y" pipe on the passenger's side. Mine is on my car so I wasn't able to offer that. Looks like you have a good plan. Glad I could help out the small way I did.

On 12/22/2020 at 10:24 AM, Dadsun said:

This is exactly what I'm in the process of doing.  I came across an old thread where @siteunseen had the very coolant hardline I needed if I wanted to avoid having to plug a bunch of unwanted ports.  I was also able to take the old manifold coolant connector off of my flat top setup and screw it into the new thermostat housing I ordered.  It was a much smaller threaded opening than in my old thermostat. Pic:

 

All I think I need now is the coolant hose reducer segment to go from 3/4" to 1/2" on the passenger side of the engine. Checking my local industrial parts store. Pic:

I know this is an old thread but these last two photos by Dadsun have me thinking.

I am in the process of getting my carbs rebuilt (1972) and doing away with the water heated manifold. (Alabama)

Why can't you just plug the nipple below the thermostat, do away with the hard pipe behind and on the right side of the motor? I plan on taking the Y-connector and modifying so it basically just goes to the heater core.

Am I missing something?

Thanks

Tad

From the 1972 FSM. I Believe this is the first mention in an FSM of coolant to the intake manifold and/or carburetor. I also believe this explains the purpose well. I don't see anything about cold or warm weather START in relation to carburetor temperature control - and why would there be when coolant is likely to be ambient temp when first cranking the engine.

I also don't know if its required to circulate coolant between the water pump to the thermostat housing if you're not going to utilize the "try to keep the carbs a semi-consistent temperature" feature. That plumbing seems to be absent in the earlier year versions. That plumbing and the "Temperature control of SU-type carburetor by engine coolant" paragraphs seem to be absent from the 1970 FSM and the 1971 Supplement. The companion "Datsun L20A, L24 Series Engine Factory Service Manual" does not contain this information either.

Edited December 25, 2025Dec 25 by cgsheen1

That coolant line goes through my 12/70 intake manifold so it's been in the cars since before the "big" changes starting in HLS30-21000. I vaguely remember in my '73 it seemed like it also looped to go through each carb which is not the case in my current car.