There are a couple of areas to look at.
Make sure the wood has no bark. Bark makes for a bitter smoke.
Cut back on the amount of wood. The model 1 is small and may not need as much wood
Try another variety of wood, maybe oak or maple
How thick, or thin was the smoke exiting the top vent? If thin, you should be good; if thick white, or billowing, that may be contributing to the bitter taste.
Is your wood wet? Old?
Hopefully some others will weigh in.