Sound like you going about it the only way you can, eliminate all the possibility's till it's right.
Could always try another carb of a HH100 or 120 that you know has a good idle.
Put the carb off the 12 onto the 10 (the one that's been giving me trouble)
The idle down issue went away, so obviously there is something not right
with the mf10 carb. Not sure what, whether it's self induced or ???
At least the problem has been narrowed down to the carb.