Strange fuel use

[BSA500]

Hi,
I had to put my spare carb on as the 1 inch monobloc slide was say loose very worn. Put on my 15/16 spare which has the benefit of having been resleeved. Now she ticks over etc. Problem is the fuel consumption, its gone from 55 mpg on the worn carb to 40 mpg. I have using it like this for months as I use her for work every day.
Same jets etc no leaks maybe running a little rich but not that rich  . Any explanation?

[a10 gf]

Hello, plugs looking ok ?

[bsa-bill]

Weaker mixture due to the worn slide (air flowing around the slide reducing suction across top of jet)
I'm no expert this is just an assumption

[BSA500]

Plugs are a little sooty not black smoke from the pipes. It may have been weaker but the plugs were brown and she went like a scalded cat but rubbish tickover.

[muskrat]

I agree with Bill's train of thought. A slightly smaller carb should need slightly smaller jets. Drop the needle 1 notch (raise the clip) and go down 1 size on the main. In the meantime go a grade hotter on the plugs (down #'s for NGK or up #'s for Champions).
Cheers