what casues a spark plug to go faulty

[bikersupermot...]

okies gilera gsm 50 supermoto all standard no mods.

runs rough, splutters an smokey esp when gas held hard open against the stop.

struggles to get to 6000rpm but above that it pulls clean an smooth.

even in neutral holding the gas hard open takes aabout 20secs to rev up to 6000, but then pulls clean an nice right up to red line.

but ... after about 70 miles with bike will splutter really badly and then it just dies.

replacing the spark plug br9es seems to fix it. for another 70 miles then same again.

electrics on the bike are working fine - so whats going on?
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[BikerMatt]

What colour are the plugs?
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So many bikes..

[bikersupermot...]

plug when taken out is dry slightly blackened. typical i think for a high revving 2 stroke - its not wet an oil - ud think it wud be for the amount of thick blue smoke it makes
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[BikerMatt]

[Hetzer]

Kittens sucked into the carbs would do it. Fur up the gap and shit.
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"There's the horizon! Ride hard, ride fast and cut down all who stand in your way!"

[bikersupermot...]

using castrol semi synthetic 2s oil n no not measured - its in separate oil tank n mixed by the bike
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[MarJay]

https://www.scificool.com/images/2012/07/9148130.jpg

Seriously though if the spark plug is 'clean' looking that just means you're running very rich and the petrol is washing the dirt off of the plug. Clouds of white smoke once the bike is warmed up is not a particularly good sign either, even on a two stroke.

Sounds like you're dumping a whole load more petrol and possibly oil than you need into the cylinder.
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[bikersupermot...]

if it was dumping more petrol and oil into the cylinder than was needed then why isnt it getting burnt and producing some power + speed?

once the revs get over 6k it runs sweet, its below 6k it splutters an smokes an has v little power it can hardly even get up hills in 1st with throttle pinned.

so i pull ova, get revs up to about 8k then slip the clutch makin sure the revs dont dip below 6k and its riding fine ... just if i let revs dip below 6k the rough running starts
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[MarJay]