Dominator 99 does not pull as it should do especially up hill, have recently replaced pistons, any suggestions

Ignition timing would be my guess.

As you have recently been working on this bike replacing pistons etc. Can I ask, did you adjust the primary chain by any chance? I had a 650ss and on one occasion I had made adjustment to the primary chain plus other stuff, and went out to check everything was ok! There is quite a long steep hill close to where I lived at the time and the bike struggled to get up it. it turned out that I had the primary chain too tight. 

The last 3 bikes I worked on for other owners all had very tight chains ,one had horrendous vibration through the footrests that competely dissapeared after adjustment ,one had a very musical tight front wheel caused by missing spacers. 

hello

are the valve tappets to tight

barry