I just did a major fuel system repair to my wife's 06 GV 250 - same motor, slightly smaller carb. I did the filter, pump and carb. Just my opinion here but it probably isn't the fuel filter - as I replaced mine with a similar basic universal filter from the auto parts store and it works fine. If you notice, while its idling, the fuel filter is only about half full of gas - meaning its never completely full or starving for fuel. It probably isn't the fuel pump as well. When mine died (fuel pump) it would supply enough gas to start and idle for a few seconds, then it would die shortly after that. Just pull the fuel line off the carb and try and start it - fuel should flow out easily. (be careful when you do this as fuel spilling on a hot exhaust is BAD...From what you've said - yours will start and idle fine until you give it gas, then it falls on its face so to speak.. The old carb didn't do this. so if you eliminate the new carb as being bad - since its NOS - it has to be the installation. My Honda ruckus did the same thing... I replaced the carb, didn't tighten the air boot screw super tight and it did the same thing as yours... started up but died instantly when given any gas. Too much air coming in kills it instantly. You seriously have to tighten those two screws tighter than you would think - to make sure you have a good seal. You're almost there bud.