WOT bogs and dies

I have an xr80r '99 and a xr50 '00 that I just bought. Both bikes ran ok but both would bog and die when you cracked to wot. The bikes were bought from 2 different people. Both plugs were carbon black as could be, telling me that they are fat(rich). Both air filters were caked, so new air filters went in. Oil change and new plugs went into both. The 50's needle clip was in stock position but the mixture screw was set 1/2 turn out so I set to 1 1/2 turns out. While carb was off, it was cleaned thoroughly. The plug looks good now but still bogs and dies when cracked to wot. The 80 had a low speed jet that was snapped off so I installed a new one(stock #35) and the needle was set on the 2nd to last notch from the bottom so I moved to stock location at 2nd from top and clean carb thoroughly before reinstalling, also checked float levels on both and were correct. Set the mixture screw to 1 3/4 turns out and started bike. Bikes start easy and idle fine, but when you crack to wot they bog and die. If you just twist throttle rather than snapping throttle to wot its fine, they both rev out. I have moved the needle clip up and down with the 80 and seems to get worse with the needle clip in the higher position(leaner), but is not cured in the lowest position(richer). Any Ideas?

I cant believe that is the way they just are, it does sound as though the accel. pump would solve the problem. Any tricks out there to get around this other than getting a carb with accel. pump? The bike is my kids and he even noticed the problem at age 9 and the first day on the bike. I would not have thought Honda would have this problem. First time dealing with the Jap bikes, not impressed so far, and I understand this is entry level stuff but that is kind of half assed, if this is true, that this is the way they are. Still would like to hear any remedies and im picking up a KTM 50SX Junior and hope it is the same.

Faceplantin, what do you mean by "Tweak the mixture screw until it is at a minimum."? Do you mean fatten it up or lean it out?

jimmy, it does it with a load, thats the whole point when my boy rides it and cracks to wot it sputters and he has to back off. If it just did it in neutral, I wouldnt be posting on here.

anymore help out there, I am getting some white smoke on initial start up until engine warms. Is this the top end going or could it be from the newly installed and oiled air filter? Could my wot problem be timing, valve issue or cam chain adjustment? I have the mixture and jetting looking good by the spark plug read and am looking in another direction to cure the wot problem. Mainly just bumpin this for more advise, thanks in advance.