Why are the bottoms of my muffins burning?

using my sister's oven ... which is at least 50 years old (imagine!) twice have made blueberry muffins after blueberry picking. the first time burnt (to me) bottoms. so the second time I adjusted the temp lower and increased the time to prevent the over browning. I also have the muffin pan on top of a cookie sheet and have them on the middle rack. the tops of the muffins and middles are fine and folks gobbled them up.
any thoughts?

Welcome back, seashanty! (referring to the avatar)
Are you cooking in a dark muffin pan? I saw a test once on America's Test Kitchen and they determined that darker pans absorb more heat, heat up faster, and give a crispier crust on things, while silver or white pans reflect heat away, don't absorb as much heat, and give a less cooked crust.

Sounds like a hot oven to me. Make sure you adequately preheat the oven and check the temp. with an oven thermometer to see if it is true to temp.

Old ovens heat from the bottom. Put the muffins at the top rack. Maybe put an empty baking sheet on a rack below that.

all good suggestions
there is an oven thermometer and it seems the temp is fluctuating while cooking after getting to the proper temp ...
you think i can go higher with the rack? maybe i will try putting another empty sheet on the bottom rack to block some heat ... will experiment to see
thanks

seashanty said:

all good suggestions
there is an oven thermometer and it seems the temp is fluctuating while cooking after getting to the proper temp ...
you think i can go higher with the rack? maybe i will try putting another empty sheet on the bottom rack to block some heat ... will experiment to see
thanks.

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are you using paper liners? IF not. Try some and see it that helps.