86 camaro overheating

I have an 86 camaro I-rocz it has a 355 in it. I did a rebuild recently on it and every since the rebuild it over heats unless i keep it in 5th gear on the highway, any other gear it overheats fast, so going through towns and stopping at stop lights makes it jump way up. It does have a carb, when i did the rebuild i replaced the water pump, radiator with a bigger one all aluminum dual fan set up, 50/50 antifreeze, at this point im at a loss as to why it would be over heating, also the airdam is fine. Any suggestions would be greatly appreciated.

Also tried replacing thermostat with another 185, tried 160, tried no thermo, all the same

I have an 86 camaro I-rocz it has a 355 in it. I did a rebuild recently on it and every since the rebuild it over heats unless i keep it in 5th gear on the highway, any other gear it overheats fast, so going through towns and stopping at stop lights makes it jump way up. It does have a carb, when i did the rebuild i replaced the water pump, radiator with a bigger one all aluminum dual fan set up, 50/50 antifreeze, at this point im at a loss as to why it would be over heating, also the airdam is fine. Any suggestions would be greatly appreciated.

check the timing if retarded it will over heat. the pressure cap must be working and NO coolant leaks. you must have pressure or the coolant will boil.

you must have the fan shroud in place , this directs air flow thru the radiator.

possible you have a bad head/gasket if exhaust enters the coolant system the exhaust gasses will prevent coolant circulation.

with the radiator pressure cap off , with the engine cold start engine see if as the engine heats up you have exhaust pressure pulsations at the pressure cap bore.