Cooling question on my 73 400

[Charger1973]

It can run in the driveway all day with no problems but as soon as I drive it down the road it gets hot.  What could the problem be? 

I have a flex fan, 26 inch radiator, no shroud and the system has been flushed several times.  The heater core is bypassed as well.  I know it should probably have a shroud but I would think that I would have problems while its idling in the driveway too if the shroud was the issue.   

[aifilaw]

sounds like your timing is off and when you put a load on the engine it is creating a lot of heat.
Also sounds like your carb is mal-adjusted and running lean

[Ghoste]

Could also be that the lower rad hose is collapsing.  Sometimes when the system gets under pressure at higher rpm they will do that (the springs inside them get old and rusty).  Give it a squeeze and see if it feels like a  new one or if it collapses real easy.

[Charger1973]

sounds like your timing is off and when you put a load on the engine it is creating a lot of heat.
Also sounds like your carb is mal-adjusted and running lean

I will double check it and the carb too.

Could also be that the lower rad hose is collapsing.  Sometimes when the system gets under pressure at higher rpm they will do that (the springs inside them get old and rusty).  Give it a squeeze and see if it feels like a  new one or if it collapses real easy.

The lower hose doesnt seem to have a spring in it at all.  I'll try replacing the hoses and make sure it has a spring in it.

[Ghoste]

A lot of the aftermarket replacement hoses don't.  If there are many miles on it, it wouldn't surprise if that turned out to be the culprit, or at least a big contributor.  You combine it with the things aifilaw mentioned and it's pretty much a lock for the symptoms you describe.

[Purple440]

I was running hot on the highway too until I increased my timing.