It sounds like the engine is going through a leaning out phase. Try opening the low speed mixture screw about 1/4 turn at a time. Wait until you replace the spur gear though. This is one reason I don't subscribe to idling the engine at all during the break-in period. If you have the mixture set too lean, the engine will overheat because of a lack of air flow. If the mixture is set correctly (which is rich for the break-in period), then the engine doesn't get hot enough. I recommend running the engine as rich as possible and driving the vehicle around varying the throttle to about half or so. Do this for the first couple of tanks, then vary the throttle up to full. This will keep the engine at about the right temps. After 4-5 tanks, you can start leaning it out for power.