OT: Petrol lawn mower engine query

What sort of starting system does it use?

If it's similar to a conventional vehicle starter, I would suspect voltage drop in the wiring or connections. If possible, try using jump leads directly to the starter motor when it's hot, and see if it cranks OK then.

It's possible that it uses the generator as a starter; many lightweight motorcycles have used this over the years. If that is the case, they only just about worked when new. They normally use a flywheel magneto, and reduction in magnetic power with age will make them perform poorly.

Chris

It's a motor with a Bendix screw, like on an old car.

My worry is that when hot I can't even crank it with the pull-cord, so something is making it more difficult to turn over.

Are you suggesting that when hot the engine it's harder to turn over by hand, or more difficult to start, ie takes more cranks?

Consider also that it may be the safety brake binding on when hotter.

If cable operated the cable may not be fully releasing the brake.

Can you eventually manage to start it when hot?

All simple engines will need more effort to turn when hot as clearances close up, and compression increases.

Chris

Harder to turn over by hand. So much so that i've never actually been able to start it, but perhaps somebody with a stronger arm could have done so.

It beats me why they don't build lawn mowers with a kickstart - legs are much stronger!

Thank you. Can you provide an authoritative reference for that please?

My experience with car engines is that they turn over more easily when hot, but that is presumably because the oil is less viscous ...

Yes. My right foot.

So do you consider a car engine to be 'simple'? Perhaps you need me to be clearer. My definition of simple in this context is a single cylinder air- cooled engine of under 200cc with a carburettor and a marginal ignition system.

IME of such engines, whether fitted to lawn mowers, chain saws, disc cutters or lightweight motorcycles is that they always require more effort to turn over when hot.

A car engine? Clearly not, but then you weren't asking about your car.

Chris

They do:

Chris

Yes - simple ...

Experience is good.

I suspect clearances, for example between piston and cylinder wall increases when hot, unless they are dissimilar materials; so I would expect compression to reduce. So it would be nice to see a reason why your experience confirms mine.

True. So what would be significant differences which would lead to my experience of car engines being easier to turn over when hot?

You suspect incorrectly. Air cooled engines in particular rattle like mad when cold because of the excessive piston/barrel clearance. As they warm up, the aluminium piston expands considerably more than the cast iron bore. They then quieten down. And the compression will then be much higher.

Although they may exist in the dim, distant past, I have never seen an engine that uses the same material for both the piston and the bore, although some performance two strokes use aluminium barrels with a Nikasil coating.

They have either roller or ball race big end and main bearings, and sometimes needle roller small end bearings, obviating the need for a pressurised oil lubrication system. The amount of drag in the bearings won't change significantly when going from cold to hot, and there pretty much will be no oil drag in a two-stroke engine.

Car engines generally are water cooled, so the temperature extremes between hot and cold are lower, reducing the need for large piston to bore clearances when cold.

More significantly, they use plain bearings that have small clearances filled with oil. The effects of all those bearings filled with cold oil are what make a car engine more difficult to turn over when cold.

Chris

Is the engine seizing up as it gets hotter? I would expect quite the opposite.

Or is the resistance due to compression, where the engine can freely spin back if let go just before TDC on the compression stroke?

The only issue I have had with hot engines is the 'flooding' of fuel.

Aha! - Dissimilar materials is obviously the key.

I understand about the bearings.

Thanks.

The resistance is due to compression, and Chris Whelan has explained why.

However the electric start appears to be weak, even when connected to the big car battery. All the wiring is good. Other electric-start mowers I know of crank perfectly OK when hot. So is it likely that the starter motor is under-powered? I know car starters have multiple brushes and multiple windings (and one brush failing is enough to weaken the motor but not cause complete failure) - is this likely on a lawn mower?

Does your mower have a safety brake to kill the engine when the "dead-man's-handle" is released? If so, have you checked that it isn't dragging?

Tim

The dead man's handle has to be operated to enable the ignition. In theory this also releases the brake. So when the handle is released the ignition stops and the brake should be applied.

But when cold it cranks pefectly OK without operating this handle because the brake has worn completely away. But of course it doesn't start. So, no, the brake doesn't impede cranking.

And as initially explained, the resistance to cranking only occurs at compression, so Chris Whelan's explanation of higher compression when warm because of the dissimilar metals of the cylinder and piston is probably fair.

The other reason for higher compression when warm *might* be the presence of more fluid engine oil helping to seal the piston rings.

Whilst it is certainly true that an alloy piston in a cast iron cylinder will have less clearance when warm than cold, it is kind of a basic principle of engine design to make sure there is enough clearance at operating temperatures. Otherwise, any trace of binding is going to cause a runaway failure, where friction heats up the parts, but the piston inevitably gets warmer than the cylinder, until it seizes.

Chris Whelan's contention was that it would have a steel piston in an aluminium cylinder because it is air cooled, and that would have clearances reducing as it warmed up.

Remember the OP's problem is that it won't electric-start when hot, and he finds it physically challenging to pull-start it.

These things are made down to a price, and my guess would be that hot- starting on electric would have been marginal even when new.

T'other was around. You always want the whizzy bits to be as light as possible.

The other reason for my suggestion was practical experience. My Son has a roofing company. They use disc cutters, and they have a very hard life. To help him out when he was starting out, I used to service and repair them for him. As they aged, they would get harder to start when hot. When they were beyond repairing, he would get a bit more use out of them by pouring water over the cylinder to cool it down.

One last thought; when you bypassed the wiring and connected direct to the starter, did you use the mower's battery? If so, it would be worth trying it with a separate battery from, say, a car. Mowers like yours usually use ni- cad batteries, and if it's the original it will be well past its best.

Chris