i was wondering what the pros and cons of covering the breather system on the top of the engine

The kleen air system to which you refer on the ZX12R, is there to add more air (oxygen in particular) to keep the exhaust gases burning longer as they travel through the system thereby depleating more of the hydrocarbons for emissions purposes.

The exhaust flow has a negative pressure at certain points in the cyclical flow which "suck open" the reed valve allowing the outside air into the port. When the pressure in the port goes positive the reed valve closes preventing exhaust from flowing back into the inlet system.

Removing the kleen air system should have no effect whatsoever on the power, although some people who have carried out this modification may tell you otherwise. It is simply done to rid the engine of clutter and save some weight in the process.

Phil

It is quite a common mod that the bike owners do to blank of the reed valves. Its all part of the clean air system which allows air to mix with the exhaust gases to reduce the hole in the ozone layer.
I left mine fitted initially but have now removed it and blanked up the covers. It makes the engine a bit quieter at idle as it is not sucking in air.
It may even release an extra 0.2567 Hp

Hellfire has beaten me to it

You need a pair of these.

Phil

Yip, removed and covered with no problems (but not as bling as Hellfires!)

Andy

Thanks for the lowdown guys much appreciated

Just a couple of other things while I remember. - put in updated clutch springs (Barnett) and check the 3 pin alternator output connector for signs of overheating. There has been a history of oil creep up the cables into the socket where it ends up shorting the phases causing the stator to burn out. I replaced mine with soldered connectors. If yours is a B1-on it should be ok but worth checking . Did you get all the sensors?

Thanks Andy, got all the bits, well fuel pump is on its way, will check that yeah its a B1

If its a 2004 engine it's a B3.

Phil