Yesterday we purchased a new weed whacker. A good one from Echo in our case.

This, like most rotary trimming tools, is driven by a two-stroke engine. These are simple little beasties with only a few moving parts. In order of appearance to the fuel:

  1. Tank (and a priming bulb which isn’t really in the fuel flow in normal use)
  2. Carburetor float bowl
  3. Carburetor choke
  4. Carburetor orifice
  5. Crank case
  6. Transfer port
  7. Combustion chamber
  8. Exhaust port
  9. Muffler

Along the way the crank shaft drives a magneto to provide the spark.

But that’s not the real point of this post. This is about breaking in an engine.

Wherever there is a metal-to-metal interface there are imperfections. (Ok, if you’re running a Formula 1 shop you might be able to get away from most of them, but we’re talking about a $160 unit here). The fuel, in the case of a two-stroke, carries with it the lubrication needed to ensure a properly functioning engine. There’s not a lot, bit there’s enough. As the parts initially start rubbing against each other they wear. They wear in to each other. The imperfections mate together to work together as a unit.

Even more than that is the piston ring. Again you have metal to metal contact, but this time you have not only friction, but you have sealing.

I really wish I had the idea to start a camera when I did this… but I didn’t.

When I first started the engine it ran. It ran pretty well considering all it had in it was assembly lubricants.

I let it warm up a few moments then slowly turned up the wick on the throttle.

As I held it there running wide open, it slowly changed it’s tune over the course of a minute. It went from sounding ok to sounding like it really came into its own.

What we were hearing is the wearing in of the interface of the cylinder sleeve and the piston ring. As that wore to be a perfect fit in our engine the sealing of the combustion chamber became better. There is less mixture blown past the rings and higher efficiency in burning the fuel/air mixture.

Listening to the pitch and tenor of the engine note made all the engineering texts I’ve read come to life. It’s one thing to know why something works the way it does. It’s another to feel it working the way you know it does.

It’s watching the machine be born and mature.

It’s pretty damn cool.