SU HD6 fuel / balance problems

Need Help. My Mark IX starts with the enriching carb (will not start without) but runs at 1800 rpm! Ran through the setup carefully according to the manual. Jets set correctly, Timing at 10 degrees BTDC. Turning down slow running screws does not slow RPM. briefly covering rear carb increases rpm. Covering front carb has no effect and seems there is no air flow to front carb. HELP

I loosen the accordion linkage clamps between the carbs so that they can be adjusted separately.

Others will chip in here ,but firstly tighten all nuts holding carbs onto manifold, and then nuts holding manifold to head, this will tighten up any air leaks entering a bad gasket.
Is your vacuum advance pipe connected to carb and dist.
Is the brake booster air pipe cracked or loose.[ disconnect and plug manifold if unsure]
Is the fast idle screw turned in, on the rear carb. [ if there is one]
Jets should be about 1/8th below bridge in venturi.
How much play do you have in your carb. spindles, move them up and down, if excessive they will need replacing and carbs need re bushing.
Disconnect the carb. spindle from each other so that they operate independently.
With engine running, screw rear idle screw OUT to reduce speed and screw front idle screw IN to increase idle speed.
Listen with hose or air balancer to see if they air sucking sounds the same and or balancer needle is in same position for both.
Once air is equal, and speed idle speed is reduced, hook up the spindle clamp.
Don’t forget to check colour of plugs, Black is too rich, white too lean, center porcelain should be a browning colour.
At 1800 rpm your timing should be about 25deg or so.
At 650- 800 your timing should be 10 btdc

Much to unpack here. Thank you. Will report results.

Have you done any recent work on the carbs or has the problem presented out of the blue?

All is new as this is a ground-restoration.

Ok, thanks. When you have balanced the carbs - ideally with a Uni-Syn or similar the air drawn by each will be the same. At this point you might find, as I did, that the sound heard through a hose at each inlet is different.
I could not get my car (three carbs) to idle even if the slow running screw was bottomed. Air was leaking around the butterfly on one of three - hence the different sound and high idle. My own fault as I rekitted the carbs. I had to remove the offending carb and reset the butterfly. Now can idle down to 500rpm. FWIW.

Thank you Paul. I can’t get to the garage till the weekend, but a quick look just now revealed that the clamp on the brake vacuum line at the rear of the intake manifold was loose. Thinking that Morris had it right! Might explain the weak brakes. I haven’t been on the road although just moving the car in the garage it seemed the brakes were weak so I did a complete brake flush (again as I had rebuilt them earlier in this decade long saga!) hoping that was the problem. It may however have been little or no vacuum boost?

Sounds like a good find to close out before going any further. Idle will definitely run away with a vacuum leak.