Hi guys I recently bought a 91 Camaro Rs 305 TBI but have been having problems with the cooling system for some reason it heats up only when the ac is on. It takes a while to heat up but when I turn the AC off the temp gradually goes back to normal. Does anybody know how to fix this?

Are your electric fans coming on when the A/C is commanded on? A/C puts a drag on the engine if the fans don't come on that may be your issue.

make sure the space between your radiator and a/c condenser isn't full of leaves and dirt, the radiator needs air flow to cool properly

Yea the Fans are working fine when I turn the A/C on and will check and clean the gap between tha radiator and a/c condenser thanks much!!

The fans should always be on when the a/c is on except at higher speeds. the stock fan really isnt a good setup...id suggest upgrading to a dual fan setup.

Or a single fan with a shroud. The main issue is the single fan setup does not bring the air through the radiator but pulls it from the sides.

How hot is hot? Too hot or do you just notice some increase? Some increase can be expected since the A/c condenser is on the high side, therefore gets hot, so then you're getting all that warm air from the condenser into the radiator.