Popcorn seems to take a long time to pop and then a lot turn out burnt
What you are describing is a maintenance and operational problem. The thermostat is fixed and made to limit the kettle to 435F. Rare that they go, and usually open. That is hot enough that if left on it will coke oil.
Fresh corn also has a big effect on the popping time due to moisture content. The machine does take 6 to 7 min to preheat before the first batch is dropped. You can turn on the agitator and add the oil at about 5 min, but wait till it is hot to add the corn. The older the corn is the less moisture the kernel will retain. The moisture turning to steam is what pop's the corn. Over time the kettle will build up a carbon layer that delays heating and heat transfer to the corn. It also will cause a carry over of heat and burning. It is very important to listen to the popper as it works and when there is no popping for 15 to 20 seconds dump the kettle. Then reload quickly or secure the heat and motor. The 2 oz oil goes in first. wait for it to get near the smoke state, then drop the 8 oz of corn and seasoning. The following is from the manual;
I've included a manual for you to review here;
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