Weber/Mikuni writeup?
 

I have plans this coming spring to replace my fb's stock nikki carb (with rat's nest intact and stock exhaust manifold) with either a weber or mikuni sidedraft. I have seen the in-depth write-ups on how to remove the rat's nest and improve the nikki, but does anyone have a detailed guide on upgrading to a new carb and corresponding intake with a basically stock engine bay? I am wondering if since I will need a new intake anyway, I won't need to go through the same process of removing the emissions equipment, ideally everything could come out at once, but that is probably not the case. I have also read up on what jets to run on the weber for certain driving styles and stock/street ports, but is there a complete list of what exactly is needed, like a new fuel pump, regulators, and all the little stuff? If only buying the right carb was as easy as getting coilovers from re-speed, you just call in and tell them what you want out of the car, and they tell you what you need..

take a look of my dellorto write up:

https://www.rx7club.com/1st-generation-specific-1979-1985-18/dellorto-48-dhla-swap-13b-12a-not-56k-friendly-773154/

I cant help you with jetting..there are several ppl on here that can help you with jetting a weber 45mm...My carb is a newer and improved weber..that was only made for a few years...or if you were looking for more of a complete kit you could always try looking at rotary shack they have an OER 45 or 47 mm carb kit. Robert is a great guy and he can also help you with jetting. or you can try putting your own kit together with a carb and just search for jetting..

Don't bother with the carb if you haven't done the exhaust yet.

Would a Weber 40 work on a stock 12A, only header installed?

nope

The exhaust isn't totally stock, but will be completed the same time I get the carb, probably custom 2.5 inch back from an rb header, the car isn't going to be boosted, so I think 2.5 would be wide enough. Nice, writeup, from my understanding the webers unlike your dellorto have no choke, so I won't have to adapt a choke cable, with the weber 48 does the throttle linkage have to be modified?

None of the after market carbs have a choke, per se. Most do have a starter circuit that works with the stock choke cable. A starter circuit eliminates the choke butterfly, what one wants in a performance carb. Some of the Weber's have neither.