YBR125 starting problem

[marksmith]

Hi all.

My girlfriend has a 2007 YBR125, with all of about 4,000 miles on the clock. It's recently developed a fault - it rides perfectly (or as well as it ever did) but occasionally refuses to start. It turns strongly on the starter button, but just won't fire. Sometimes it may give one little "chug" of a fire, but that's it. Leave it an hour or two and it starts perfectly on the button.

It's only done it a few times so it's hard to characterise accurately, but it doesn't seem to be related to engine temp - it's done it from stone cold, and after being ridden and then left for a few hours (on a warm day).

The bike has a kick-start but unsurprisingly when it's playing up that doesn't work either. Absolutely confident it's not the battery though - it continues to crank strongly as long as we can be bothered to keep trying.

I've replaced the air filter and spark plug, which has not helped.

My bike's in for some work next week, so I'm going to ride this around all week and see if I can get it to fail for me. I could do with some tricks up my sleeve for when it does.

I'll check for spark. Any other ideas? Leaky injector flooding the engine?

Thanks,

Mark