Alright, here is current jetting for my '83 IT490 runnning a yz piston and a DG IT490 pipe.
Main jet= 380 (it pinged with a 370)
Pilot jet = 40
Jet needle = 6DH4 with clip in center position ( the standard needle an 6F8 needle did not work as well)
Needle jet = RO, 247 series (you must specify the series when ordering)
The best place I found for ordering mikuni jets is from Pats small engine parts. They have good stock, ship fast and prices are low.
The standard carb slide is a 2.0. The YZ465 slide is a 3.0. You should at least try the 3.0 slide, I think this is important. A common modification is to open up the 3.0 slide to a 3.5. You can do this with a file if careful. Search the yz490 group of yahoo for details on how to do this.
I did the needle jet, jet needle and slide modification pretty much together. My bike ran much better with these jetting changes. It now pulls cleanly off the bottom end and hammers right to the top. It is also easy to start so long as you kick hard from the top of the stroke.
While I am here, I will offer an opinion of gearing. I found the stock 44 tooth rear sprocket too small for tight woods. I went to a 48 tooth and found this to be more suitable. I was still able to use the same chain with this change but not with a 50 tooth.
The 48 let me get through very tight areas without needing to clutch it to avoid a stall. These things do not have the lightest pull clutch in the world, so this helps save energy over a longer event. In fact, replace all the cables if they are not fresh. Also check and lube the brake actuators in the brake plate. You really want all the controls to work as smooth and as lightly as possible.