<VV> Engine Problem

[Smitty Smith]

Smitty Says:  Ok guys here is another one for you.  (Why do I get all the nutty ones)   Engine has always been quiet and runs strong.  Now at a little over 100K miles.  On the highway at 80 degrees and 60-70 mph it starts missing and backfiring.  Mostly out the tailpipe.  Very ratty and ragged as one would run with bad plugs under full acceleration.  But only when HOT.  Engine shows no signs of overheating.  It has received new plugs, wires, points kit (rotor,condensor etc), cap, coil, points plate and replacement (proper) distributor.  It is not carbs.  If it is not ignition then it has to be mechanical.
  I have one theory, now lets hear yours.  Last "O" ring job was over 10 years ago.  I wonder if something has worn to the point that some of the valves are just barely closing.  Thermal expansion could be causing them to go less than fully closed when the engine is fully hot.  Like I said, lets hear yours.
  Oh Yeah, I have enjoyed the cow jokes concerning Lon as much as anyone but when he is up and around again I am sure he will get tired of it before the thousandth one is laid on him.  Be considerate.