Old gas & how does a lawn mower generate spark & do timing?

[Nick Cine]

Took apart the pull mechanism of my single-cylinder Craftsman push lawn mower today since it hasn't been used in a year and it wouldn't start in one pull and the recoil spring mechanism was not recoiling after the first pull (the cord just hung limp).

When I cleaned up the recoil mechanism & put it back together, I found some of last year's gasoline, and it worked just fine (so anyone who says you can't use old gas is gonna have to explain why it worked just fine for me today).

After two or three pulls with the choke locked open (otherwise it slides shut too quickly), the motor started and after about ten seconds on choke, I removed the clamp I put on the choke lever to keep it in place.

I cleaned out the air filter too and cleaned up old grass in the blade area (and a bunch of rope tangled around the driveshaft - I guess slowing down the blade is what makes the motor shut off since I defeated the shutoff mechanism due to it being a PITA as you always have to hold it on).

Anyway, my question is I started to wonder how the spark is generated, and I wondered how a simple (Craftsman) lawn mower advances its timing.

[hubops]

I was just about to reply with the definitive lecture on this -

- but noticed that you have the rare and exotic single cylinder push mower ! ... all my expertise is on the multi-cylinder models. Sorry. John T.

[hubops]

youtube can sometimes be your friend.

Old gas isn't _always_ problematic ; very old gas _is_. In general, spark comes from the coil, activated by the magnet on the flywheel. In general, the "governor" is ruled by the fins on the flywheel - working against a small spring on the flapper that controls the carb .. John T.

[micky]

That controls the carb, as you say, giving it more gas iirc when the engine slows. The fins blow less on the flapper/sail which no longer overpowers the spring, which then pulls the throttle farther open.

But there is no spark advance on a the small lawn mower engines I worked on. Of course I only substantially took one apart 60 years ago, and less so 20 years. ago.

[Snag]

It uses a basic magneto , a magnet on the flywheel and a coil to generate the spark . Timing is static , there is no advance .

[Snag]

It's called a "magneto"

The governor controls only engine speed , has nothing to do with spark timing - which is static , there is no advance mechanism . Study up John , it appears you don't actually know how either system works .

[AMuzi]

Difficult question about fuel. First off, there are various 'gasolines'. Large metro areas have a very different blend (aka 'EPA gas') and summer fuel is different from winter fuel blend. And California is different again. Generally I've found no-ethanol fuel more stable for storage of a year or even two. Ethanol fuel can also grow mold in fuel bowls and lines after sitting a long while.

In short, draining the system is good practice before storing an engine but not draining it may not be a serious problem.

[hubops]

I didn't mention spark advance at all - just where the spark comes from and the carb governor. John T.

[Snag]

You oversimplified to the point that there's no meat on that sandwich . Usually a mark of those who do not understand the systems they are talking about .

[hubops]

... and you changed magnet & coil to magneto. Thanks Einstein. John T.

[Clare Snyder]

Spark is produced by a flywheel magneto - later ones have electronic trigger built into the coil - old ones had points and condenser. Timing is fixed on MOST smalkl engines. The odd one MAY have elecctronic spark retard designed into the coil trigger circuit