Idle RPM changes with engine temperature.?

I'm getting my '87 Toyota Pickup ready for smog - 2WD, 4 cyl, carbureted. I drive the car for about 10 minutes to get it warmed up, set the timing mark and idle RPM as stated on the emissions label. It idles a little rough but is okay. The strange thing is that as I leave the car idling and it continues to heat up, the RPM keeps increasing. The truck runs fine, but for the smog check I have to be within 100 RPM of the stated 700 RPM on the label. Within 10 minutes, it had jumped from 750 to 850. I guess I can set the idle at the smog shop just before I turn it over to them, but it still might creep up while they're working on it. Any suggestions?