[timthefern]

1978 L250 ac and auto trans just put new carb on. engine now purrs BUT the idle speed rises and falls through a cycle of about 3 minutes very gradually decreasing by about 300 RPMs then climbing back up to about 800 rpms.
The AC is Diconnected and the idle stop solenoid is unplugged is this a vacuum issue? Please Help? could it be a fuel pump issue? I am leaning toward a vacuum issue because the rise and fall of the idle speed is not random it is very regular. Again, Please Help?

 

[62 NovaWagon]

I'd check for a vacuum leak first thing!..

 

[Norfolknova]

Also check fuel pressure at the carb. My 73 was doing that and it was getting way too much pressure to the carb causing it to flood out.