2005 kdx 50 problem

[tfdladder1]

I need help, my kdx 50 is not running correctly, the problem is that the bike runs fine in nuetral but when put into gear and given fuel it has no power, sometimes backfires, and jumps forward a few feet at a time. The weird thing is that if you take pressure off of the back tire by lifting the bike, once it gets past the first 1/8 throttle it runs fast. I have cleaned the air filter, carb., put in new fuel, changed jets and needle positions all with no help.
I am now thinking its possibly something in the ignition timing, any ideas would be helpful!