When you say...."The two outside valve push rods seem to be as tight as possible but they are not closing properly"
The valve cant close if theres absoulutely no free play/clearance between the pushrod, rocker arm and valve stem. That will cause low compression, valve burn out, backfiring and total failure to run depending on how over tight its adjusted!!!!!!!!!
When the one valve (Intake or Exhaust) is open the other must be closed so on that closed valve the adjustment has to be loose enough so theres like 0.020 clearance between valve stem and rocker arm WHICH ALLOWS THE VALVE TO FULLY CLOSE.
When you say the pushrod is always tight it sounds like theres absolutely no free play lash at all. Get the other valve open then loosen that always tight valve so theres a 0.020 gap between the valve stem and rocker arm
Orrrrrrrr perhaps I dont understand what you say or your question???