I friend gave me a snapper I524. It runs, but only on full choke. While it runs it surges and occasionally tries to die. If I hurry up and push the primer button it will keep on running for a little while. It's been in storage for a couple of years so I'm thinking the throttle body assembly might need a little TLC. I'm having trouble getting the cover off. I've taken off the top hex nut by the muffler and the two screws on the side of the cover. The knob for the choke is in the way. The knob is connected to a thin shaft. I pulled on it a little and twisted it both ways but I don't want to put too much muscle into it and break it. What is the right way to remove the knob? or is there something else I'm missing?