It was originally in because it wouldn't start (ironic). When I picked it up, I started it outside the shop first pull.
When I actually had time and the grass was dry enough to mow two weeks later, it wouldn't start.
The repair shop replaced a damaged throttle cable and spark plug as well as a few minor parts (handle bolts etc), and did a service. I had been having issues with the throttle cable for a while and had been manually pulling it back and clamping it to the handle to keep the mower running. Obviously not a great, or permanent solution.
After adjusting the pinch clamp it started immediately and appeared to run the whole time on the same revs, no black smoke.
I guess I know I need to take it back to the shop, was trying to avoid the hassle of starting work late or finishing early. Also, I like learning how things work, however maybe I should stick to trees.