81 300td - loss of mpg...

Went from ~22 down to ~19 - plus sort of a rough idle. This is a rebuilt engine with about 10K miles (according to the previous owner, but the rebuild was also about 8 years ago and the car was sitting in a garage).
Bought the car in March, about September noticed the mileage going down and a seemingly rougher idle than I remembered.
Any ideas?
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Assuming tires are correctly inflated, this engine should deliver about 24 mpg in 15 to 20 mile commute driving, some freeway some city.
If tires are correctly inflated, I suggest you adjust the valves and add injector cleaner to the fuel.
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Thanks! I'll have to roll up the old sleeves and try to figger it out!
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