We just use 4 bathroom scales.
The first night went decent for my daughter. For her first night ever racing, and only driving the kart up and down a friends driveway a couple of laps she didn't finish last place. The kart was a little loose in the corners from center out, but she was able to hold it. About halfway through the feature she lost power. The engine was still running smooth, but she couldn't get up to speed. I found that after the race if I spun the rear axle by hand it would only turn about 1 turn and stop. One of the bearings was hard to turn. I am suspecting that the bearing would get hot and cause too much drag slowing her down. I replaced the bearing and the rear wheels spin free now. Tomorrow night the two of us will go out and check the valve lash, coil gap and check to make sure the carb doesn't have some junk in it. The kart had been sitting for about a year, so it is possible it has some buildup. We will also scale it and see if I can get it to stick a little tighter coming off the turns.