Symptoms of an Intake Leak?

If the idle jumps up that means something might be out of adjustment and the car is telling you it wants something different. Check your carb linkage/throttle return, make sure that works as it's supposed to. If it does not return all the way then you need to fix that first.

Like said above, check for an intake leak with carb cleaner. Spray around any suspected leak areas and listen for a change in rpm. If it goes higher, there's your leak.

Once you verify all the simple stuff, maybe take some initial timing out then re-set the carb to the new base line and see how it goes. Verify fuel pressure and float level before you start messing with the carb.

Could be a blown head gasket though, maybe just starting. 240 is too high. Notice any white smoke from the tail pipe? Might want to check your oil for milky color. When it's cool, take off the rad cap, start it up and check for bubbles in the coolant as it warms up. Have you noticed any coolant loss but no external leaks?

What kind of cooling fan do you have? Got a shroud?

Also could be a stuck valve. If the gauge is bouncing, that can indicate a valve issue. Might be a Guide, spring, rocker or the valve has carbon on it preventing it from sealing etc.