Try this, it might work. With the engine running carefully, don't burn yourself, turn the shaft to the left and to the right. The engines exhaust should be trying to "open" the valve and you should be able to feel that the flapper wants to open one way or the other. If the shaft doesn't want to turn spray it down with WD-40 but the engine has to be cold otherwise the WD will catch fire and that would be a bad thing.