I have 1999 KTM 300 EXC why dose it bog out when I am in first three gears and puff a lot of smoke ? But as I go through the higher gears the smoke deminishes and has no power band ?

SOURCE: Smoke

If it's liquid cooled then that grey smoke is more than likely steam ( the head gasket is on it's way out). If it was burning oil you would able to smell it.,,

SOURCE: no power band

Are you sure you havent just kinked a hose somwhere. Maybe the crankcase breather pipe or even the fuel tank breather pipe.

If the timing was okay before you stripped it I really would double check all the pipes.

Also if you had the engine upside down when you were working on it perhaps some oils got on the plug so preventing you getting a decent spark

SOURCE: i have a 91 ktm 300 exc and it bogs down and dies

Your carb needs to be cleaned. More specifically, the pilot jet, but if it's clogged it'd be best to go ahead and clean the whole thing.

Remove the carb from your bike and then remove the bowl from the bottom. You'll then be looking at the jets and float valve. The jet causing your issue is the smallest one. Remove it and carefully spray it out with carb cleaner. Then hold the jet up and see if you can see light through the hole and that it appears clean and round-meaning no obstruction left. Locate the mixture screw on the outside of the carb. It should use a flat screwdriver. First turn the screw in and count the number of turns. Then remove the screw, followed by the spring and ,if applicable, a small o-ring. Spray through the hole and make sure it goes through and comes out somewhere. Replace the screw in the reverse order and set to the original setting you counted in the beginning.(rough estimate is 1.5 turns out. Cleanliness is the key to the carb working. So keep it spotless inside and reinstall. This should take care of your problem.