Under load the spark plugs need more voltage than while cruising, so make sure the ignition is capable of producing a good hot blue spark at least 1/4" long. My slant pole original coil gives a good 1/2" spark.
back firing usually means a lean condition, fuel running away from the jet in the carb on steep inclines, you might have your float set just a little low, try raising it just a little, don't raise it so much the carb floods
It could also be low on gas. If you have bellow an 1/8 tank and try to go up a long hill you may start running out of gas. All the gas goes to the back of the tank instead of down through the outlet.