while performing the tuning of my su carbs i noticed a black metal tube that goes underneath and between both carbs. I believe this metal tube was originally connected to one of the down exits of the air filter orange box. Is it worth removing or is it doing something there? Could it be a source of heat transmission and add up to the heat soak problem during traffic jams? I see the black paint is peeled and does not look good!

That system is to put hot manifold air into your intake to stop the carbs from freezing.   I would say you can safely remove it in Panama

Have other users removed it? it looks ugly by the way! Whats the name of that metal tube? I hope it does not affect the choke time at the moment starting the car. What i mean is that the car will need more choke time to warm up once i remove this metal tube!

No it will not affect choke time.

Did you removed it? please explain the purpose and name of such part! why was it originally connected to the airbox filter?

http://www.gremline.com/index_files/page0032.htm

Yes, remove it.  Its the pre-heater for the airbox to be used in colder climates (look at the summer/winter flapper on the orange airbox).  This is what it looks like once its off.  It probably isn't doing any harm, but isn't doing any good either for your situation.

Several other items to check on your quest for idling without RPM dropping...

 

How is your fuel pressure?  Have you actually checked it with the engine running once its warmed up?  I plumbed in a cheapo gauge and am getting A steady 2.0 GPH out of my electric pump, which is right where it should be for carburetors.  Could your pump be pushing more fuel then your needle valves can support within the float bowls & making the carbs run ultra rich at idle?  At cruising speed, it wouldn't be noticeable as the fuel is getting burnt at higher RPMs.

 

How is your engine vacuum?  Again, plumbed in a cheapo gauge into my balance tube & am able to confirm the health of the engine very quickly at idle and higher RPM.

 

Reference https://smile.amazon.com/gp/product/B00GM5FPXO/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

checked my fuel pressure last time and it measured 3.5 psi! No vaccuum leaks anywhere!

No vacuum leak, but rather a health check for the engine.   You can very easily tell if you have a leaking valve, carb's out of sync, broken ring, bad timing, blown headgasket, etc.  Please see below:

 

 

This is a video of my 240z once it was tuned & running perfectly (post engine rebuild).

 

VacGauge.MOVVacGauge.MOV

Looks like the hose to the PCV valve is shot. That might cause a vacuum leak and interfere with PCV function.