I have an 84 with a 305 4bbl carb...The car has been running just fine, I used it a couple of days ago without an issue...I haven't used it since Friday and now I start it, it runs fine, but after about a minute the idle jumps up...it goes from about 900 to 1200...any suggestions?...ideas?....Thanks....

 

Im guessing a possible vacuum leak....

...You could have sticky throttle plates... What Carb is on it? Rochester? or after market? It could boil down to just restting the carb for summer .... Could also be a intake leak.... How many miles on it?

 

it's the rochester...it has 147k miles on it...i noticed something...there's a rod that goes to the carb from I think maybe the EGR or something on the driver side that has a black rubber plunger looking thing on it...when I pull that rod out, the idle stays down...is that the EGR valve?...

 

More than likely Its vacuum dash pot for the secondarys...

I would replace it..It sounds like the rubber diaphram has worn out via a crack or a hole in it...causeing the secondarys to open ever so lightly causeing the idle to climb...thus allowing the throttle plates to open just enough to change the idle.

Since that model year Rochester Carb has Vacuum controled secondarys.
That should solve your problem...

 

great...now let me check autozone to see if they carry that part...thanks again....