Do you use your fuel shutoff when your done with the tiller? For some reason if you don't these little Honda engines tend to leak gas into the oil thinning it out and overfilling the oil so it blows it out the carb. Drain your oil and and see if there is gas in it then replace with new oil and see if it still happens. The main thing is to use the fuel shutoff every time you finish with it, good luck.
I have a Honda 2000 watt generator and a Honda 5000 watt that had the same problem. When I went to change the oil in them there was a strong smell of gas and the oil was very thin, also both oil filters were wet with oil. I changed the oil in both and started turning off the gas and problem solved. Some times it's easer and cheaper to start with the easy fix first. AndyM's problem may be all together different but it's worth a try.
