'86 2WD, manual

Major loss of power going up hills. I used to be able to go 60mph up a decent hill in 4th gear, now I can barely go 35 in 3rd gear.

Things I have replaced and checked:

Cap and rotor
Spark plug wires
Timing adjusted
Checked for air leaks
No error codes showing
Fuel filter was replaced about 2 years ago, 10k miles

Any ideas or suggestions would be appreciated. Thanks