Dakar 650 hoes en proes when riding at constant speed.

[express]

Friend acquired a single spark 2003 Dakar 650.
When accelerating pulls well but when coasting (constant speed) feels like it is missing.
What can be the the problem ?

 

[JourneyMan]

 

[JourneyMan]

 

[JourneyMan]

 

[JourneyMan]

 

[Smuggy]

 

[Wildcoast]

 

[Jughead]