I have a '78 242 w/ a B21A engine. It starts quickly when cold but when I go somewhere and return before it has completely cooled down, it takes a long time to fire up. What is causing this?

Its your auxillary air valve. It isn't closing once the car is warmed up. On a B21 you must have CIS. FCP Groten has them for $56.00. I just changed out mine on my '75 242 with the same symptoms and all is well now. Before ordering check that you are getting voltage the existing auxillary air valve. If there is no voltage check your fuel system relays

Common problem with carbed cars. Vapour lock. The heat of the engine vaporizes the fuel in the lines causing a lean condition. You need to give it gas while turning it over.

With fuel injected cars the gas remains under pressure when the car is shut off so no vaporization (if there are no leaks).
--
1980 245 Canadian B21A with SU carb and M46 trans

I do give it gas when I'm turning it over. What else could be the problem?

Carb tuning may help. If you've got an SU carb, see
http://www.brickboard.com/RWD/index.htm?id=763761

I run on the lean side for fuel economy but there's also less of a chance of flooding from giving too much gas and I can use the choke if the engine revs start to drop (stumble) because of heat caused lean condition.
--
1980 245 Canadian B21A with SU carb and M46 trans