OT: Petrol lawn mower engine query

Graham J · 2017-07-30T07:03:40+00:00

Mower is flymo venturer turbo 420xl probably about 15 years old, which I've recently repaired by replacing the failed magneto coil and cleaning out the rusty crud from the carburettor. History is that it was used for some time (perhaps 3 or 4 years) then failed to start so was abandoned, left in a garden shed. I jury-rigged a car battery to start it while working on it, so it will crank for many minutes if necessary. When cold, it starts with a couple of seconds of cranking. Also, I can start it by hand with the pull-cord. It runs well, and I've used to to cut the grass. When hot, the electric start struggles to crank the engine. It either stalls at compression, or turns over so slowly it fails to start. This is with the big car battery! By hand, I'm not strong enough to pull the cord. If I let it cool down for 30 minutes I can start it by battery or pull-cord. Does this indicate an incipient failure? Or is it to be expected of this sort of simple engine? The crankcase has oil up to the mark. I haven't replaced the oil but it looks reasonably clean; I've no idea whether it's ever been replaced. There's no evidence of an oil filter. Any ideas ...?

Jul 30, 2017 27 Replies

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Graham J 9 years ago

[snip]

All testing was done with the jury-rigged car battery.

Surprisingly the original NiCad isn't dead. I'm told it never worked, but part of my investigation revealed a corroded crimp joint inside a moulded connector. The magneto has an alternator coil attached, see:

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... and the corroded joint was several hundred ohms but allowed the NiCad to charge while the motor was running. So I was very surprised that the NiCad registered over 12v even though the mower had not been used for several years.

Having charged it fully it does now very nearly crank the engine when cold - probably this is very nearly as good as when it was new!

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Tim+ 9 years ago

Presumably then it was relatively easily started with the pull starter when both cold and hot in the past?

So what's changed that makes it so hard to turn over now when hot? Presumably clearances haven't got tighter within the engine over the years? Just the opposite I would have thought.

Tim

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Graham J 9 years ago

Good question. In the past pull-start when cold was OK. But when hot, I don't know - I may be able to find out. The mower belonged to my neighbour - now dead, and was used by the old chap who cut the grass for her.

More recently a younger chap brought his own mower because he could not make the neighbour's mower work.

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newshound 9 years ago

Excellent point

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Kellerman 9 years ago

Take the spark plug out when its hot. Can you turn it easily then? Yes? - compression - are the valve clearances ok when hot i.e. are the valves actually opening? Is there excess fuel or oil in the cylinder? No? its trying to seize up! :-)

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Graham J 9 years ago

[snip]

Plug out - cranks easily.

The valves are opening. No excess fuel or oil. Once started, engine runs smoothly with no exhaust smoke. I've used it to cut the grass - it runs fine for half an hour at a time, no problem, no funny noises.

So it must be increased compression when hot. I did have a compression tester, but I can't now find it ...

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