carb rpoblems

[Cray]

ok heres the deal, i got a 77yz250 & 77 yz400. both of them run good but a little "blubbery" as i call it. just not crisp accelleration or good top end. but if i turn off the gas valve with the engine runnin after it drops the fuel level in the carb to almost nothin it runs like a demon, has twice the power and accel.. i know the float height is not to high i have checked it many times on both bikes. i killed the engine as soon as the fuel level got low and it stated runnin good and pulled the carb to check the amount of fuel it. it was damn near empty, id say about 1/2 inch off the bottom of the carb.
so heres my question, why does it run so much better with almost no gas in the carb than at normal level. ive done this over and over with both bikes and they both react the same. could i be way over rich with the jetting? they both still have the stock size jets in them.
im stumped

 

[125 rider]

Yes it sound as if you are running rich. What you described also occurs because when the engine is running out of fuel it starts running lean. In some circumstances lean = more power up to a point but there is a difference between optimum air/fuel mixture for power and optimum air/fuel mixture for safety. Start jetting leaner until your plug is a nice mocha brown and your response is without hesitaton. Also, dont make it a habbit of running your bikes out of fuel.