Ok, be easy on me, I'm new to this forum and I'm sure this question has been asked dozens of times. I've had street and dirt bikes on and off most of my life so I know a little about them. I just bought a 2001 KLX 125 that is overall in good shape. Supposedly it has a new top end. My question...the bike bogs down and loses all power to the point of cutting off when's there's any resistance on the rear tire. For example, if the back tire is coming out of a steep ditch or if I'm going up a fairly steep hill, the bike will lose power in 1st gear and will actually cut off. It's almost as if I'm trying to climb the hill in 4th gear. Even with a running start / momentum, the bike loses power going uphill. It's frustrating as sh** and even dangerous. My son has a KX 65 and that thing can climb a tree. It never loses power on anything !! On flat ground, the 125 seems to run great and has plenty of hole shot and power. What am I doing wrong? What is the bike doing wrong?

 

Resistance to the rear tire occurs on a hole shot too. Does this happen only when climbing a hill? If so, could this be a carb float height issue?