The Hyper21 8 Port is a great engine.I have one in my Savage and love it!! When you turned the carb around did you push hard on the carb before you tightened it down?I didn't push it down tight and it was leaking air and caused an erratic idle like you are describing.The O-rings need to seat right or they will leak.Also put some locktite on the backplate.Mine came loose after about 15 tanks.Take it real easy on the pullstart too.It isn't cheap to replace and it's not very strong.I'm on my third one in about 4 gallons.Until they make something like the Roto Start to fit it your stuck with the pullstart.