The YZ 426 is an amazing bike!
... 5 valve head and a pretty awesome cam of the shelf, (which is why they backfire on deacceleration
In standard trim they do run hot, however you could unleash a lil power and allow it to run a lil cooler, (particularly around the exhaust port
), simply by taking out the baffle, AND
removing the "header restrictor"
, (remove the silencer in it's entirety an look into the header pipe where the silencer assy. attaches, you will see what looks like a washer welded into the pipe ... GRIND IT OUT!
After doing the above, you'll probably need to re-jet it ... I'd suggest going up one size on both main and pilot to a 165-170 main and 42-45 pilot ... Your air screw will be somewhere between 1 and 3/8 to 1 and 1/2 turns out.
Wrt your air box ... it stands to reason that if you unplug the exhaust, you need to do the same to the inlet ... removing the rubber "trumpet
" on top of the airbox should allow it to flow a lil more air, without exposing you to any undue risk of water entering the airbox.
Finally on plugs ... contrary to what a lot of people think
... you want the COLDEST
possible plug that will run in an engine without it bogging, and as a good rule of thumb, you can safely drop between one and two notches on the heat range!