Oh, I forgot, if the engine is "missing" when rolled inverted, you probably have the mixture set too lean on the ground. Try richening up a couple of clicks.

If that doesn't work, then check over the muffler vent line to make sure you don't have a pinhole or some such in there. Also, check the clunk as mentioned above.

On many airplanes, the tank is set a little higher or lower than "perfect", and the result is often a change in mixture setting when rolled inverted. If the tank is too low, the engine can go rich inverted, and it can go lean if the tank is a bit high. Some trainers have the tank mounted high for ease of engine tuning and with the assumption that negative G's aren't going to be a common occurance for that plane.