Petrol lawn mower dead

Strip it all down fully, or buy a new one? Can you get new gasket sets, so you can reassemble it if the internal damage is not too bad?

My experience of Briggs and Stratton single cylinders is around 45 years out of date but, as fitted to garden rotavators of the day, they had an adjustable ignition setting. First rule is to lay the plug on top of the cylinder head and see if cranking gets you a spark.

If the blades are mounted on the crankshadt, you have bent it and the motor is junk

WHS

At a guess: Flywheel key broke. There is a an alumin(i)um key connecting the magneto flywheel (on top of the engine, with the pull starter) to the shaft. This shears off by design (that's why it's soft) when you hit something, to protect the crankshaft (which is sometimes doesn't protect).

Sheared, the timing is off, and nothing works; replace and done. (Crankshaft bent? -- ouch.)

got to

and read:

"Lawn mower will not start after the blade hit an obstruction" and following.

Thomas Prufer

"A mower that won't start after its blade hits an obstruction may just need one or two 25 cent locking keys replaced. "

sheared pins wont make it stiff to turn over

I’m wondering if the clout has damaged a bearing in some way.

Thanks very much all for your help and suggestions. I have now got to the flywheel and sadly the flywheel key looks intact and the flywheel notch and the notch on the crankshaft look properly aligned. With the blade in place (no belt) and the spark plug removed, I can turn the flywheel easily for 3/4 of a turn but the remaining quarter requires a bit more force and it "sticks" in parts. Anything else I can try or does it confirm that The Natural Philosopher called it from the start and it is a bend crankshaft?

Thanks

Lee.

That too, but its classic bent crankshaft behaviour. No one puts hardened steel into a lawnmower crank.

You can buy a working model of this mower for around £50 on ebay Or an new one for a couple of hundred.

How do you value your time?

A bit of an update. I saw a YouTube video of someone suggesting to check if the crankshaft is bend by pulling the starter cable (with plug removed :) ) and watching the centre nut of the blade to see if it rotates about the same axis or moves around as it rotates. I did this and couldn't see any noticeable issue. I then put it all back together (braking the little plastic fin attached to the carb!!) and it started first time! However that is a lot of vibration when it runs. Could this be because the the plastic fin isn't regulating it (it does sound fast) or is this more confirmation that something in the engine is amiss?

The crank is more bent than David Hockney was. New mower or worker from ebay/gumtree etc. and make one nice mower out of the two you would have.

The little plastic paddle is part of a rather crude speed governor arrangement. The paddle gets blown by the airflow against a spring in a direction which causes it to close the throttle. You get a sort of dynamic equilibrium where the speed-dependent airflow leaves the throttle open just enough to maintain the desired speed. If the mower encounters some resistance - e.g. from long grass - the engine slows and the throttle is automatically opened a bit more. Without the paddle, the engine will overspeed until the valves bounce - and may destroy itself. Can you not DIY a replacement paddle?

Replacing it will be an easy thing if we think that is the cause of the vibrations. Given the previous issues wondering if that is the cause of the vibrations or something more sinister.

Have you unbalanced the mower blade by hitting the obstruction?

Worth taking it off and sticking a pencil through the middle to see if it is balanced, and also check for chips in the cutting edge.

Cheers

Dave R

I agree - but the quote is extracted from that web site

Assume you have nothing to lose and bend it back?

One video bending it straight, using very basic tools.

Thomas Prufer

Ah good idea Dave. Will check it later