alright quick question...i got a yoshi rs3 offroad slip on pipe for my z400 bue when i put it on theres a patter noise in the motor....its not there when the stock silencer is on just the yosh...what could it be?could it hut to run it?

 

Most times when you put on a less restrictive exhaust you need to rejet. Not exactly sure what you are hearing but it may be pre-ignition from a lean condition, if it is in fact lean then yes you can damage the engine severely. (Engines are like real life - better to be a little on the rich side).
A less restricitive exhaust also advances your timing a tad, unlikely there is a timing adjustment so you may need to change the gap on your sparkplug, that also will change the timing and can compensate a little. I believe making the gap wider retards the timing. You may have to go with a colder sparkplug too. (If it is lean you go colder if it is rich then hotter, if lean then you narrow the gap if rich then widen the gap) In your case it is more apt to be lean. So for you it is sort of a 'catch 22' situation.
Most times just rejetting gets you dialed in pretty close..
I guess you want more power because you changed your exhaust. This means rejetting and changing sparkplug heat range and also gap, to get your timing right, and to burn your fuel correctly to get the performance out of your exhaust mod...