My sons Scrambler 90 will cut out and die when ever he goes up a slight hill, runs great on flat ground. any suggestions??

 

The obvious answer seems to be a problem in your carb. I haven't messed with a 90, it doesn't use a float type carb does it? But don't rule out more obscure things like the gas tank vent line being clogged. My 400's vent line fell on the exhaust and melted together. It did ok on flat ground because I wasn't getting on the gas much there because of tight trails. I see you have a 400, so you know you can't hold the throttle open long unless you have plenty of wide open spaces to let it fly. But on hills I got on the gas more and it caused a vaccuum in the tank that starved it of gas. My bet is on a problem in the carb, but check everything that has to do with fuel flow.