I took my mower to Mt G Mower Centre after I had replaced the carby diaphragm and gasket and then found that the idle speed was very high. I was told I needed 1. oil change 2. airfilter 3. new blades 4. new throttle cable 5. carby service 6. new spark plug - quoted $178.
I said no thanks and paid the $25 quote fee.
After a few more Youtube videos, I found that I had to properly reconnect the spring from the carby to the speed governer. A 5 second fix - mower now runs perfectly. I wanted to support the local business - they have lost me.

Approximate cost: $25