When I was about 16, I wanted to build a go-kart. And not just a gravity-powered seat, but a proper go-kart with a gas engine. Where I would have driven it is a mystery, but I started the project anyhow.
After a trip to a car scrapyard and amazing luck, I came away with a welded go-kart chassis that someone had left, and a three-speed gearbox. The chassis even had a working push-rod steering wheel. Somehow I also ended up with a dirty 3 horsepower Briggs & Stratton engine. The engine didn't run, but I had never seen a lawnmower engine that couldn't be fixed by replacing the spark plug and/or cleaning the air filter.
Over that summer the project took shape. A friend and I cut some plywood to size to add a floor and seatback, and my dad helped by donating two dollies which I dismantled to get their wheels. The only components missing were the engine and drivetrain.
Meanwhile, that stubborn orange engine had been refusing to even cough. Knowing the fundamentals of internal combustion are air, fuel, and ignition, I went through these one at a time. The correct answer is often the simplest one.
The air supply was perfect, but I gave it a new filter. The fuel tank was rusty, so I bartered with a neighbour for a broken lawnmower in his garage with a shiny gas tank. I replaced the spark plug with a new one even though the old one seemed to work. In the process of testing that new plug I got my first (and only) 20,000 volt shock.
Ignition was definitely the issue. There was no electrical current to make a spark, and this was where my troubleshooting ideas ran out so I bought a small engine repair manual. Finding a suspect wasn't hard; the ignition coil is the source of the spark in those engines.
It had been months since starting the go-kart project, and winter came. I have a dad who understood and let me repurpose the tool shed as my workshop. With a small space heater and a 100W trouble light, being there was pretty enjoyable.
I dug into that engine, past the fairing, past a wire mesh, past the flywheel key, past the flywheel itself, and then past the first layer of the crank case. At each step I cleaned everything and hoped I could put it together in the right order. Finally the ignition coil was uncovered, and replaced.
All the parts went back together fine, except for the obligatory "extra" ones.
And the engine still didn't run. I don't remember just how bad it was - whether the spark was nonexistent or just too weak or intermittent - but in the end I was definitely disappointed.
I also don't remember exactly why or what part it was, but there was something I felt needed adjusting, down near the ignition coil. By the time the summer came again, I had opened the engine all the way to the coil multiple times, and got pretty quick at it.
Finally, one day I put it back together, pulled the cord, and it started and stayed running. It was loud, but it ran smoothly and cleanly. I remember being confident in it because I knew all the pieces and I knew that they were all working correctly. There was no room for gremlins anymore.
The go-kart itself fizzled out. Looking back I was too preoccupied with including the gearbox in the design, and didn't stay focused on the simpler engine-only solution. However, everything gained was probably worth the time anyhow: the knowledge about small engines, the sense of organization and ownership that came from that "workshop", and that feeling of joy and relief when the engine finally ran.