Spark plug or battery?

[rk8479]

Yesterday this happened

I was comming out of work and tried to turn on my bike it made the sound of comming on but didn't actually come on,

After 4-5 attempts the bike would do it anymore (with each attempt the bike sounded weaker and weaker)

I called the RAC and they couldn't fix it so they recommend I change the spark plug and recharge the battery,

Later that day I did both if the things he said, I charged the battery up to 12.8 volts and when I tried to start the bike it took a few attempts but it did come back on so I turned it off and left it for the night,

This morning I tried to start it and it wouldn't start so I checked the battery and it read 12.0 volts so I went inside and left it on charge for 4 hours after that it read 13.05 volts so I went to my bike fitted it up and tried and tried but still no start and it also got weaker and weaker after the 5th time

Can someone help me with anything as what might be the problem thank you

My bike is a 4 stroke 125cc scooter

[Kickstart]

Hi

Sounds like the battery is on its last legs. A half decent battery should manage to crank the engine over for quite a while and shouldn't be losing charge that much over night. Surprised the RAC bod didn't just jump start the bike.

Anything else slightly wrong (such as a spark plug that is a bit old) will make it a bit less likely to fire up quickly, and with a battery on its last legs it might not manage to start before the battery has had enough.

All the best

Keith
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[rk8479]

Yeah he hooked it up to a device that gave it energy while he tried to start it but it still wouldnt start so he looked at the spark plug and diagnosed it as not having a good enough spark to start the engine so yesterday i replaced it with a new NGK one and recharged the battery to 12.8 volts and then the rest of the story on the first post happened

[Ariel Badger]

You need a new battery, end of.
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[Teflon-Mike]

The problems are related.

Poor battery means that there's not enough power to spin the motor over fast enough, for long enough to get it to start.

Old Spark-Plug, will mean that it needs more electric to spark between the electrodes, and weak spark will mean motor will be less inclined to start at lower cranking speed.

Ie: you need to change BOTH, plug and battery.

Worth noting that volts is electric 'pressure', what matters here is amps, which is the 'amount' of electric flowing.
Batteries are rated by Amp-Hours, which is basically how many hours they can flow one amp, or how many amps they can flow for an hour.

Little 125 probably has a battery rated some-where around 6 Amp Hour or 6Ah. So you get 6 amps for one hour or one amp for six hours from fully charged, at 12v.
If the battery is old and duffed; what usually happens is that the plates inside the battery get corroded up, while the acid gets weak and the level drops. Less surface area on the plates, means they dont hold the rated amount of charge, so your 6Ah battery might only hold 1Ah or less, even though volt meter says it can still deliver it at 12v or there-abouts.

Your starter motor can draw as much as 30A.. if you have a good, brand new battery, fully charged and delivering all it should; that means that battery could keep it spinning, for 6/30 = 1/5 of an hour or aprox 10 minutes. But obviousely if its not a good battery, that is going to drop a lot.

Next up, spark plug. The ignition system makes a spark by taking volts from the battery or generator, and with a clever 'transformer' that makes the 12v supply voltage higher, but does so reducing the current draw.

You need a lot of volts to get the current to jump the gap between the electrodes in the spark-plug, hence get a 'spark', BUT for that spark to set fire to the petrol and air, sucked into the engine, you have not have enough 'energy' which is volts times amps; volts alone isn't enough.

So, the spark plug has to have the right 'gap' distance between the two electrodes. If its too big, it will need more volts to get a spark accross it, and further you make it jump, less energy its going to have. Too small a gap? Spark will jump easily, but wont pass through enough petrol to set anything on fire, or will set such a small fire, that it wont get it all burning fast enough to get a good 'bang' to push the piston down.

Meanwhile; every time the spark-plug sparks, the 'spark' erodes a little metal off the electrodes; so the gap grows. you also find that it erodes more metal off the corners of the electrodes, as that is where the sparks preffer to jump from; and you get deposits build up on the surface that try and block the sparks in, or can, if the plug is whats called 'fouled' offer a 'short circuit' around the gap, so sparks wont jump the gap.

This is why engines like nice clean new spark-plugs fairly often.

And on small bikes, the power supply is often the generator, not the battery. Seems a bit of a daft arrangement, but the battery supplies the electricity to turn the starter motor, which turns the engine; turning the generator, that makes 'different' electricity to make the sparks... seems a long way around the job when you have electricity in the battery, but there is a techical reason for it, to do with the timing of the spark and getting that big voltage for it.

BUT means you do have to have a good battery to get the motor to spin up quick enough that the generator makes enough electric to make the spark... and you need a good spark-plug so that it doesn't get wasted.
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[arry]

What charger are you using?

If you get something like the Optimate 4 it gives you a visual of battery health when you plug it in. They're expensive ish at £50 but so far I'm really impressed with mine; simple plug in and it chooses what charging method is best for the battery.

Would recommend getting one even if you do get a new battery.

[rk8479]

Thanks guys, ive ordered a new battery.

im just surprised it managed to get me to work but just seemed to loose it after, i was only gone for an hour

[_Iain_]

[Walloper]

[_Iain_]

You realise those prices are actually cheaper than what we buy them in at trade for?

Feck knows how they do it, unless ching chong fakery rather than genuine NGK
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[Walloper]