This 'B' carb is on the 207 ,, right ? ,, so the main jet is ample big enough , a 50 would be more economical .
Drilling in the carb ,, NO WAY ! maybe push a little fuse wire thru the passages and wee holes , nothing violent though. Compressed air and carb cleaner solution is my preferred tools . These are metered holes , carefully designed , and dont take well to tinkering , end result gross fuel consumption . Does the condition clear as the engine warms up ? Is the heat riser box working ?
You have checked the carb to manifold gasket , and the manifold to cyl head ?
As Tony says " The power valve is normally in the enriched position,until engine vacuum closes it,the little piston moves freely?The small tight wound spring is present?"
You say it accelerates fine over 2000 rpm , , an electric fuel pump will not change that .
This is a part throttle problem , which means an emulsifier tube or power valve problem ,,
or something completely different (like the vaccuum advance) but I don't think so .
