The symptoms were very like the engine running too rich: the motor starts to sound rough, then starts emitting smoke, then coughs and dies. The spark plugs were sooty. So I took a good look at the air intake and carburettor, and fished out the piece of rubber I found inside it. Put the whole lot back together again, and hey presto! Nothing. For good measure I replaced the fuel pump, but to no avail.
So I took it all apart again, took the carb off and cleaned it. Some of the little channels were blocked with powdered grass, so I cleaned them as best I could, not having an air jet. I couldn't get the float chamber screws off but I tested it by blowing in the fuel intake (yes, air goes in) and then turning it upside-down (yes, air stops going in) Possibly not very healthy, but it worked, and it also confirmed that the main jet isn't blocked. Put it all back together again. Still no va.
I was surprised to see the spark generating mechanism: a magnet mounted on the flywheel passes two coils (one per spark plug) so the timing is fixed, and the spark doesn't depend on the state of the battery. There is a spark in both plugs, so that's not the problem.
Fuel is getting into the cylinders because I made sure the float chamber is full by putting a funnel on the fuel line, and also because I can smell it when the engine turns over. I'm a bit stumped now, and the situation is not helped by a feeble battery that only just turns the engine over, and then only for a short time. I've been jumping it from the car, but that doesn't work for long either.
Might have to call in a pro. Rats. Meanwhile I'm cutting the grass with the trusty walk-behind mower. It takes a while, but at least it works. Seems a bit heavy on the back, for some reason.
It's always worth cleaning the plugs and resetting the gap if you've not done that already. Even though you get spark it might not quite be hot enough
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