There is an existing question asked a while ago - What are monsoons caused by? and I just want to know whether it is better to ask a more concise version of this question restricting it to the Asian Summer Monsoon and answer it as well OR answering the existing question as is while limiting myself to the Asian Summer Monsoon alone.
You should ask a new question specific to the Asian summer monsoon. The problem with editing the old question is that narrowing its scope to a specific region may be considered a major edit and those are discouraged if they change the intent of the question greatly (which I'd argue would be the case).
Whether the old question is too broad or low quality is an issue independent of your proposed question. If that old question is kept around, one option is keeping its current scope and making it a canonical monsoon question where a single community wiki answer points to other questions specific to a monsoon region such as the question you want to ask.
I would wait for other opinions on this before acting, but, I believe an answer focusing on a particular monsoon could be useful - it could prompt others to post informed answers about the other monsoonal systems.
That user wrote 81 questions and seemingly left - not answering requests for clarification.
Alternatively, the question could be considered as too broad and closed as such, in which case a focused question and answer as you are proposing could be a good option.