Joolz,
did you read the ramblings which accompany that photograph?
Lack of response to the idle mixture screw is closely associated with the secondary butterfly not sealing properly...

Have to confess I hadn't until 5 mins ago. The substitute carb is still on the engine at the moment, but I tried the oil trick with the carb on the bench and it took about 10 minutes for a drip of oil to seep around the butterfly and reach the paper towel it was sitting on. I expect the test will give more useful results when the carb's on a running engine, with the oil either temporarily sealing the small gap, or being sucked straight past.

The butterfly certainly looks like it's sealing, but I'll try backing the screw off a turn and see what happens.

Joolz,
normal manifold vacuum on an engine at idle speed is around 18 to 20 inches Hg.
Since 2"Hg is about 1 psi, that 20 inches of vacuum is approximately 10 pounds per square inch between the inside of the intake manifold and outside air pressure.

Bearing in mind that any vacuum leaks will have the most noticable effect at idle, the only worthwhile test is with the carburettor on an engine that's running.
Time to suck it and see?
http://www.mgaroadster.co.uk/Technical_ ... %20See.pdf

p.s. Finally got my hands on one of those Chinese? copies of the Solex 26/35 a few weeks back.
Not a lot to write home about, the owner was most unhappy about the significant air leak I discovered at the spindle for the secondary butterfly.
Considering that it had only been on an engine for a month or so, it was in worse condition than most genuine Solex carburettors with over 100,000 miles use.

So although I haven't put the carb back on the engine, I'm not hopeful that adjusting the secondary stop will sort it. As I said before, the carb looks to have very little wear on any of the other moving parts, the car it's from is showing 40 something thousand miles and judging by the overall condition it could well be the first time around.

I gave the screw a tweak with the carb on the bench, and it only needed backing off 1/4 of a turn before the screw lost contact with the stop. So assuming the screw should remain in contact, or its pointless, I left it where it was.

At a bit of a loss now, I blew through the idle mixture passageways again, from the base of the primary emulsion tube chamber, to the outlet on the base of the carb with the L shape recess. That is right isn't it?

I'll try it on an engine again when I've got a chance, just in case it's fixed itself while I wasn't looking.

before you fit it, clean the bypass drillings by teasing out a few copper filaments from a piece of electrical wire and poke through the small passages just below the 1st butterfly then blast through with carb /brake cleaner Clean any crud off the face upon which the 2nd butterfly screw rests.

Modern fuels cause white salts to form when the water laden elements start to dry off and react with the zinc alloy of the body. Solex made Alcohol carbs for VW that had a coated body

http://www.thesamba.com/vw/forum/archiv ... 066--.html

Are those the same things as progression drillings, there are 2 of them and they look like they go into the same passageway that the idle mix comes from?

yup and because the crap travels from the larger drilling to the tiny drillings they are a choke point for all that lurverly white fluff that develops in carbs if they dry out.