TW125 starting problems

Hi,

I have just bought a tw125 as my first bike. Once I get the engine running it runs perfect...getting it started in the first place however is a different story.

When I press the start button I can hear the starter turning over but it dosen't start. Sometimes I can get it started by persevering with the elec start and other times I need to bump it. Once it has warmed up it starts straight away.

I have had a look at the spark plug and when I first press the electric start I get a spark, then nothing and then when I let go of the starter I get another spark. I am very much a beginner when it comes to this sort of thing so should I be getting a constant spark when the starter is pressed?

I have tried replacing the following:

Spark plug, spark plug cap and battery. I have also serviced the bike since I got it.

I wondered if it could be an earthing issue so I tried running an additional earth strap straight to the engine block but this doesn't seen to have helped.

Does anybody have any ideas???

Thanks for your help!