Chainsaw Repair

I encountered a strange problem today while trying to adapt AM chainsaw carbs on brushcutters
When started up with H and L set to standard setting the engine was revving to around 5000 rpm. Backed off LA until it fell off! No difference. Tried pressing down the throttle, nothing. Finally I opened up L by a full turn and the idle dropped, but running like it was about to flood.
I have had this issue on some carbs earlier and I had given up on them. But this time I did a through check.
No this wasnt caused by air leaks like worn out seals. The culprit was that miniature passage on the throttle butterfly that comes on many carbs.

Single carb picture with pink circle shows AM carb butterfly with the passege
So, I used my soldering iron and lead to fill that passage, filed the excess off and Voila! Problem solved.
Picture of single carb with green circle shows AM carb after the soldering.
Why do some carbs have them and others dont?

But now the carb works fine on a FS120, then tried it on a FS55, still runs smoothly. Adjustments work very well. Right now the only mod was to move the impulse from the external tube type to the inbuilt to match the inlet manifold, plus the throttle valve. I have four ready. All that is needed now is to fit the throttle shafts from old carbs depending on which model needs a carb.
I forgot to mention that this carb is working fine also on 180 and 170 chainsaws. At about the equivalent of 6 USD , they are a steal and the extra work worth it