Carbs...Sometimes maladjusted carbs can create a hellhole of issues..Its worth eliminating that from the equation..
I have had the same problem before in a different Z when I was very new to these cars and it was adjusted by a professional mechanic (whom I think was more familiar with Carters and Holleys than imported dual SU carbs...Anyway this might not help, but what I did was to go to basics and find out as much as I could. Why should I pay somebody to screw them up when I could screw the process better…: So, (with the assumption that the floats and timing are correct.) based on the information we had available at the time...no internet available.....I went for it and here is what ended up being my outcome procedure for various Z’s that I ended up owning. It always worked for me. I have seen people suggest or do the procedure a bit different (missing some of the steps or do it totally different), so perhaps my way is wrong, but my cars screamed with a passion and I didn’t have any issues with shifting 1st, 2nd, or 3rd at 6000+ rpms or idling all day cold or hot at 600 rpms.. They pulled with all the power they could muster. Good old days…
Tools: Screwdrivers, tooth brush, carb cleaner, clean towels/rags, paper towels, lubricant (cable?), paperclip, RPM meter, Airflow meter, SAE20 motor oil. Lots of patience is always required. SU carbs are not footballs if you get upset….;-)….
1-Pull all spark plugs to see if any of them could provide any clues based on their appearance (front vs back ones), I only use NGK plugs if I can help it. Used some Bosh Platinum before and they did ok, but I think NGK may be a bit smoother.
2 checked my timing and set it properly if possible. Always CHECKED for vacuum leaks on all vacuum points/hoses including on the timing advance hose, manifold, brake boost line etc.
3-Checked fuel pump/pressure (I like to have a filter, a pressure regulator and a pressure gauge on the supply side, and a valve and clear filter on the return line to offset spikes/lows and for visual checks) NOTE: SU carbs like a constant 3.5 psi minimum to operate and try not to go over too much if possible. High pressure can overpower the carb floats and then you run the risk of unmetered raw gas into the engine. Check/change fuel filter.
4-Now for the fun part: Took the carbs apart and laid them on a clean towel. NOTE: DO NOT MIX parts from one carb to the other.
5-Cleaned my carbs with cleaner and a clean hard bristle tooth brush...nothing harsher, no sand paper or green cloths etc.... Used paper towels to dry them or you can use compressed air...inspect for abnormalities and a secure and straight needle.
Ensure that you clean out the oil/damper tube to remove any particles/gunk etc that may be present. Dry with compressed air or naturally.
NOTE; Do not strech nor compress the piston springs by hand. Handle them carefully, you may want to compare their lengths and also follow AK260's suggestion of measuring their compression, however do not damage them in any manner. Just cleand them off with carb cleaner and let them dry by themselves or use compressed air.
6-Backed off ALL adjustment screws: fast idle, balance, and butterfly valve.
7-Turned the fuel mixture on the bottom until it the JET was EXACTLY leveled with the top of the brass jet casing or whatever you call it, where the needle goes into.
8-Using a bent thick paperclip, I would use it as a gap gauge to gap the butterfly valve, then I would turn the screws exactly the same quantity one full turn. NOTE: What I turned in one, I would turn in the other...no deviation...
9-Would put the piston back in very carefully, then I would put 20 Weight Oil in them, then the spring. There is a low and high mark on the dipstick/damper. Fill it to the high mark. I only use this one: 3-in-1 MOTOR OIL SAE 20 available from Grainger. It has a blue label and specifically says SAE 20 MOTOR OIL. Do not use the black label ones that say multi-purpose, nor would I use any other types of oil, transmission fluid, etc. If it is not a SAE20 MOTOR oil, I will never use it.
10- With the pistons, springs and cap in place, I would set the screws on the cap carefully and then lift the pistons with one finger all the way up slowly to make sure they were not rubbing and getting stuck, then i would tighten the screws ever so slightly. Then do the lift on each one again 3 times: every time, it should go up slowly with resistance from the oil and when you remove your finger it should fall with an audible clunk. Both carbs should have the same sounding clunk, if not then they are not dropping equally and most likely are not going up equally either. Sometimes the housing is not set properly and it causes friction on one side which is reflected on the operation of the piston. This and a visual inspection of how the needle sits can help tell you if the needle is bent. Once everything looks normal, tighten the screws appropriately and recheck the up/down action of the pistons.NOTE: Don’t forget to check your choke cable and if it binds when released and inadvertently keeps the gas mixture setting rich instead of letting the assembly fall back into its set point..
11-Now after the carbs are in move the choke assembly under the carburetor manually (jet gland) down and back up. It should always come back to rest exactly even with the jet casing. Once established that it is not sticking (I always lubricated my assembly) then it is time to adjust it. Use cable lubricant to lubricate the assembly and the choke lines. Should not use grease since it can pick up dust, dry up and become sticky.
12-Move the MIXTURE knurled knob or wheel 3 full turns on each carburetor, then back off exactly one 1/4 turn. The knob/wheel should now be where the jet resting at the equivalent of 2.75 turns from the top brass casing. Member AK260 has suggested using a caliper to measure that. I think its a good idea and how I would incorporate that would be to do the turns manually, then check it with the caliper to obtain a base value before making the rpm adjustments with the car running, and write it down on the car logbook, Then once I had finished fully adjusting the carbs, then I would take the dome off, get a final reading (assuming that I had adjustments to the mixture and get a running depth reading and write t also down. That way, on my next carburetor adjustment event I would already have the final reading that I could use to start off with...Just an idea but I liked AK260's suggestion.
13-Connect a RPM (tach. dwell) meter to engine. Have a “Uni-Syn” carburetor flow meter ready. Heck, I still have both my meters plus a timing light from over 30 years ago… They should be useful on my new old z…
14-Now try to turn the engine on. If it does not start, turn both butterfly screws ¼ turn EQUALLY to increase vacuum in the carbs. Repeat until engine starts and holds.
15-Adjust both BUTTERFLY SCREWS to have an idle of 600 (manual) 750 (automatic transmission), max depending on engine condition. There may be a difference if the engine is tight vs. worn out almost at a rebuild stage.
16-Adjust BALANCE screw SO THAT when one side moves less than a hair, the other side also moves. Once they are even proceed to next step.
17-Adjust the FAST IDLE screw to barely touch the assembly.
18-Rev engine, let it fall down, then check the front carburetor flow. Do this quickly and adjust flow meter adjustment so the indicator is on the half way mark. Check it again quickly, don’t kill the engine. Rev engine between all checks and always let engine fall back to idle before checking again…
19-Now check the rear carburetor to see if it has the same level flow. If so you are half way there. If not, then ADJUST the BUTTERFLY valve to increase or decrease the carb flow until it is equal to the front one.
20-Once they are equal use the FAST IDLE adjustment to bring the engine to 2500/3000 rpm. If your engine will not do that, do at least 2000-2500 rpms.
NOTE: At this point, rev your engine several (3) times, note where the rpms come to idle each time, if they are stable and come to rest at say 2500 each time, increase the flow of gas by ADJUSTING the jet knob/wheel ¼ turn only. This must be done EQUALLY to both carbs. If there is not a difference, then go an additional ¼ turn and repeat. There will be a difference in RPMs, either they go up or they go down. If they are going up, continue with the adjustments until they start going down, then UNDO your last increase adjustment. This will identify the correct setting for your particular engine/carb setup. Depending on engine condition your initial increase may actually lower the RPMs. If so do the test in reverse by lowering the gas mixture instead of increasing and undoing your last increment when the RPMs decrease.
21-Re-adjust your flow meter opening to compensate for the new increase in flow.
22-Rev engine several times, let it fall to your new idle. Test FRONT carburetor, adjust flow meter to have indicator on the middle mark if possible, if not use other marks. Rev engine, and RETEST FRONT carb, rev engine, TEST BACK carb. Do this several times to ensure that if there is any difference between the two, it is consistently the same difference.
23-If there is a difference, ADJUST THE BALANCE SCREW ONLY either increasing or decreasing its effect. Continue to test and adjust the balance screw until both carb flow readings are even, then you are almost done. When they are even, proceed to the next step.
24- Release the fast idle all the way out. Rev engine. It should come to rest at the intial set point of 600/650 rpms. If not, adjust the BUTTERFLY SCREWS EQUALLY to bring it to that point.
25- Now adjust your FAST IDLE screw until you have your desired idle rpms. Some people like the 600, others like it higher, but most seem to keep it between 600 and 900 from what I remember. I like to see an engine at 600 and stable. NOTE: These settings are for a manual transmission. Automatic transmissions the idle is usually 750 instead of 600 but you can always do the adjustment for 600 and then just set your FAST IDLE at 700 rpms. If your car is manual and it has been properly adjusted and the engine is in good shape, it should be able to idle all day on 600 rpms. Exceptions; Any engine mods (cams, etc,) and if you have air conditioning, you may want to set it a bit higher, just in case you want to be in the parking lot at idle running the AC unit or stuck in heavy traffic on a hot day....
DISCLAIMER: This personal procedure assumes that before you start it you have checked or are confident that the FLOAT are within specifications and the TIMING is correct, and therefore only addresses checking the SPARK PLUGS, VACUUM LINES, FUEL PRESSURE & FUEL FILTER, then adjusting the Carburetors accordingly. Don't forget to check the AIR FILTER(s). They will change the final Misture composition if they are not flowing air freely. Another thing to consider is not sucking hot air from the top of the engine compartment but to utilize the original aircleaner box and get cold air from the front of engine, preferably from the other side of the front wall. I know that a lot of people like the individual air filters like the WELLAND and others but what you are pulling into the engine is hot air sitting above the headers....You can utilize a prefilter that just keeps the rocks/debris from getting into your air box. One thing to remember is that you always adjust the back carb to the front carb, not the other way around. Anyway it is a matter of preference so I am bringing it up as a thought.
Clarifications: Mixture screws are on the bottom of each carburetor, Butterfly valve screws are on the back of the carburetor and control how much the butterflies are open at idle, balance screw is on the center balance assembly and controls how the rear carburetor is affected by accelerating in comparison to the front carburetor, fast idle screw is the top idle adjustment screw, JET is the movable tube (that receives the needle) that moves up and down within a fixed brass casing.
Anyway, sorry if my crappola suggestions don’t work for you, but I figured I would pass on my measly useless info in case it had anything you might find useful…There is a lot of information out here that may help you more than this but I figured I would add my .02 cents.