i have an 05 trx 450r, with a full yoshi trc exhaust and a 11.5-1 weisco piston, k-n filter with and open top breather box, im pretty sure tha cam n everything else is stock n it seems to be running lean im getting a greyish color deposit out the end of the pipe n sum pretty good backfire when letting off the throttle. My question is what size jets should i be running on this thing as i dont want the valves popping off n goin thru the piston it was that way when i got it n hadda put a new head n piston in it. any help would be greatly appreciated as im still tryin to learn this bike... thx...

Do you know the jetting in it now?
by adding those type mods that open up the air flow to the engine,, it's a general rule of thumb that your going to have to richen the carb a bit..
I would start with adding two or three numbers to the main,, and one or two on the pilot.

Here's a good way to tell if you bike is running rich or lean.
Get a set of jets,, and a box of fresh spark plugs.
Preheat the motor,, meaning run it for a awhile to get it to normal temp..
Now quickly install a fresh plug and run the bike up to wide open, then quickly shut off the engine.
Save that plug and write down what settings the carb are at..
Change the settings in the carb and repeat the run cycle,, save the plug and write down your carb settings.
Do this until you start getting a overly rich condtion and pick the best looking plug.
Use a same spark plug just for preheating the engine to it's running temp.

tha numbers i got off em r 175, 72, 58, n 32 in know tha main is tha 175 but no sure of the rest, im not to good with carbs just yet lol... thx for the help...