Good news!
As you altered the float height noticeably you will want to check if the mixture is still correct by lifting the carb pistons by 1-2mm while the engine is hot and idling. If the engine does not start to stumble (lean) or runs noticeably faster (fat) it is correct.
As a fairly greenhorn to Jaguar engines, I always wonder how rich those engines run when cold. That aux carb seems to dump a huge amount of fuel into the intake.
Has anybody measured A/F ratio when operating under cold start?
On one of my client’s cars I discarded this aux carb and added a cable instead to drop the jets, causing the engine to run rich enough.
I never calculated it, but it’d not surprise me if it were 10:1.
They are adjustable of course, screwing the brass nut around the needle down leans it out. Needs to be rich enough to start but not so rich that it stalls after a short while before it switches off.
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Here is the adjustment nut that Randall described.
It limits the downward movement of the needle when in operation,
i.e. how much needle is exposed here.
There is a little spring on the side to keep it from rotating due to vibration.
I should have shown that within the body the needle moves through a fixed jet drilling that extends this high, to the yellow line.
One would have to examine each car model to determine whether this is above or below the fuel level controlled by the floats.
On a 120 it appears the level is below the top of the jet. The only exposure to vapor would be that little port on the side and a bit around the needle.
Also, the nut doesn’t really tighten, that’s what the little spring steel keeper is for. Have seen these without the keeper permitting the nut to move around. Like many fuel adjustments this one is very sensitive. FWIW.
Logic says it needs ~250% more fuel just like other engines, the xk likes and tolerates rich mixtures so it will be at least that.
I don’t know the real numbers but assuming it needs 3-4l per hour when hot it would need maybe 12l when cold and likely a lot more?
A glass full of fuel per cold start is something I can live with!