So you've decided to toss in the towel and install Weber 38 carburetor instead of Solex 4a1 on your engine. Good or bad decision - hard to say. That said, you probably will have some trouble tuning it as the installation instructions from Redline (the official Weber distributor in the US) are lacking. Here are some helpful pointers.
1. Idle stumble can mean that your mixture is too LEAN or too RICH. The engine doesn't like too little fuel or too much. Best way to tell is to pull a few spark plugs (1 from front and 1 from back). If the ceramic tip is brown - you are ok, if it's white - you are too LEAN, if they are black - you are too RICH.
2. Get yourself a vacuum gauge (they are dirt cheap) and see where you stand with vacuum. There are 2 nipples on the carb: 1 for ported vac (on the side close to the engine) and 1 for manifold vac (the curved nipple near the accelerator linkage). Connect to manifold vacuum and see the pressure. The vac gauge will come in handy later while tuning the carb.
3. Get yourself a timing light with tach meter (again, they are cheap) and see where you stand with timing. Make sure you DISCONNECT vac advance as on your car (M110 engine, right?) it's a retard so at idle it retards the timing, thus produces less vacuum, thus less mixture gets sucked into the engine. Sounds familiar? When vac advance is disconnected you should time you engine at 8-12 BTDC (manual calls for 7 BTDC static). See where your MECHANICAL advance takes you (on mine the springs were very stiff and I would always get way less advance than I should). The more advance (within the specs) the better.
4. Fuel pressure. All Weber literature asks for fuel pressure of no more than 3.5psi. Your MECHANICAL pump probably outputs around 5 so you will need fuel pressure regulator set to around 3psi. Some will also mention fuel return but a knowledgeable Weber expert said not to worry about return.
5. You should also check valve clearance as all those (timing, valve clearance, mixture) co-interact but if it's too overwhelming just temporarily move on to tuning. Just be aware that when you finally do your valves you will have to re-tune the car.
A list like the above scares the crap out of any novice mechanics (it did scared me when I started) and they mistakenly move to tuning assuming all the other elements somehow will fall in place. Since you are dealing with a very old car - no matter how well taken care of - it will have out of specs issues here and there so it's always good to confirm that all the elements are within specs.
TUNING:
You have 4 screws on your carb: 2 mixture screws (at the bottom of the base), speed screw (near the linkage) and fast idle screw (hidden near the choke).
MIXTURE screw controls how much fuel is allowed to be sucked into the engine at idle (turn right = less; turn left = more fuel). Remember the engine cares about "perfect" amount of fuel NOT more. More isn't better!
SPEED screw controls the RPM of the engine when idling (when engine is warm). Screwing in this screw you open the throttle plates exposing the transition holes (secondary holes above the mixture screw holes). Ideally when tuning the transition holes are totally covered (you can confirm it by connecting your vac gauge to ported vacuum nipple on the engine side of the carb - it should read close to zero). Covering transition holes will drastically reduce your RPM during tuning but it will also remove the variables as you are dealing only with the MIXTURE screw hole.
FAST idle screw deals with cold start and choke - specifically the RPM at cold start. It controls how far the throttle plates open when choke is engaged at cold start. As the engien warms up and the choke disengades, so does the fast idle screw and the plates close to the position controlled by the SPEED screw.
So...
-- Warm up the engine and remove air cleaner for easier access (final adjustments MUST be done with air cleaner on as it restricts the air movemnet - carb adjusting screwdriver is a must to adjust the screw close to the engine = expensive!). This will also let you confirm that your choke - the metal plate near the carb's top entrance - is fully open as it should be open with warm engine operating at regular temperatures.
-- You screw both MIXTURE screws in all the way until you touch the bottom (don't over do it) and back it out 2 full turns. I mark one side of the screw as I often get confused counting - yes, counting to 2...
-- You screw out SPEED screw until it doesn't touch the cam anymore and screw it in by 1/2 turns (I like having my vac gauge connected to PORTED vac to confirm that I don't get any vacuum from that nipple = transition holes are not exposed). Speed screw is best set with carb inverted so you can confirm that the transition holes are not exposed. Make sure when you set your speed screw the choke is off and the fast idle screw is NOT engaged! Besides the fuel/air mixture delivered by the carburetor, at idle engines need some air to breath. On many carbs there is an air bypass screw that controls just how much extra air is let it, but it isn't so on Webers. The only way to let in more air is to open the throttle plates, thus exposing transition holes. With transition holes exposed you won't be able to set idle mixture, so the only way to let in more air is to drill holes into throttle plates (pic below). Needless to say this is a one-way road, so only do this when you know your carb very well and know that there is no other way to properly set your carb.
-- Start the engine, it might sound like crap.
-- Start screwing in the MIXTURE screws by 1/4 turns (on both sides - as each screw feeds only 3 out of 6 cylinders - remember that perfect mixture on one side doesn't make the engine sound good if the other side is out of balance). You can later fine tune it by using 1/8 turns. Basic idea is to get the best sounding / most vacuum producing engine. Many will tell you to use vac gauge connected to MANIFOLD vacuum and tune for highest vacuum but in my experience the vac difference is so small that you truly have to be an expert to perceive it. Use you ear.
-- When you find the best MIXTURE screw settings, back out the mixture screw slightly (1/8 to 1/4 turn) to make sure your mixture is slightly on richer side then move on to SPEED screw.
-- Turn in the SPEED screw and set you idle at around 900 RPM (using your timing light with tach meter). I'm certain that it will involve exposing the transition holes more than one should which means that you might need to re-jet your carb. Again, there is no "proper" idle speed - rather what you are comfortable with. For instance on my M110 engine with automatic transition, when engaging Drive, idle drops by around 300 RPM (to 600 from 900 mentioned above). At 600 RPM in D, the engine doesn't sound so wonderfull so I personally up the idle to around 1000 RPM so in D I get 700 RPM - just personal preferenace that costs me a bit more fuel.
-- Drive around and confirm by pulling the sparks that you are not too rich or too lean.
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