my brother in law has a blaster with a YZ490 motor in it, and I have a buddy who had an '82 YZ490 years ago... the fix to the way it runs involves engine work... oddly enough, I also found an article in one of my old Dirtbike magazines(nov. 1986) not long ago about how they fixed their YZ490...
this is what is printed in their article...
"there is an ugly cough off the bottom and the machine blubbers like it is jetted too rich, but when you try to lean it out, it detonates"
"the best modification you can do is to have the head recut and the squish band matched to the piston. The problems are the gap between the piston and the head, and the angle of the head in relation to the piston. Race Tech performs surgery on the head, which in turn allows you to properly jet the bike and use lower-quality gas without fear of major detonation."
"Race Tech ported our cylinder. Why port a cylinder on an open bike, you ask? Race Tech believes in a controlled and smooth powerband, especially on an open bike, where violent surges equal loss of traction and lack of control. They dont do anything radical when they're grinding, but say it's an important step in gaining the perfect flow from the bottom rpm to the upper hit. Combine the head mods with the porting, and your 490 accelerates brutally hard but has no hitches in the powerband. The last job Race Tech performs is to cut the carburetor slide to their own specifications and install a set of boyesen reeds.This is an important step in the jetting and fuel-flow process."
I'm in no way trying to talk up race tech here, its just who they used to do the work in this article... but it should still give you some ideas of what you need to do...
I'm sure there are quite a few places who know what needs to be done to the YZ490 motors, and how to do it...