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CFM 80 PG conv. Just fixed the problem with the furnace shutting itself (not detecting flame) turned out to be a bad relay in the brain wich I replaced.

I am worried now the furnace may be over heating. After comming home and turning up the thermostat from 69 to 71. The air comming out of the vents topped 184 degrees. The furnace wall around the burner assembled was at 280. The high limit switch was not open. I tapped on the silver disk and it tripped the high limit switch and shut off the burner.
Don't know if the high limit switch is working. Does 184 degree air seem excessive?

I would say that 184* out of the vent is WAY too hot I believe it should be in the 120-130 range. I would start with the High limit switch.

Greg

That's high but there's another problem you're not catching, not moving enough air through the furnace, hole in the heat exchanger, blockage in vents, etc. Did you ever replace the blower motor? Was it the right one? Has the heat exchanger been inspected? That's an OLD unit. Is the asbestos pot (combustion chamber) still intact inside the unit? High limit sounds bad and should have kicked the furnace off. BUT that unit should never get that hot unless there's something wrong.

Replaced high limit switch.
The funace was purchased last year (blower included) as a hole re-conditiond unit from local service center. I was wondering about that. The blower should move as much heat as the furnace produces. And come to think of it my old furnace did reach the high limit switch regularly too. I just thought it was normal. The only commen denominator then would be my duct system. Maybe blocked.

The combustion chamber appears good and clean. The heat exchanger, although I have not sean it gives no indication of a problem.

My fault...they are still available. Just don't see any "newer" ones in my area. Use a mirror and flashlight to see if the duct work is clear. Are the fins on the floor registers bent closed? Pull them all out for a day or 2 to see if that helps. Any chance there is a solid door as access to the furnace alcove? Leave it open to see if that helps. Either way there is not enough air moving through your furnace. Look for any restriction of air flow be it demand (ducts/registers/etc.) or supply side (air to the blower).

Does it trip the new high limit switch?

The unit could be oversized for your ductwork and home, or as DaleM says, there's a restriction. A blockage that large should be noticeable, as-in not much flow coming from vents. If the flow is adequate, then the burner/heat exchanger should be the next place to have inspected.