I know this is off topic for a mustang forum so if the mods decide to delete this thats fine, but i know there are some very knowledgeable folks here that i would like their input on this problem.

I have a 79 bronco with a 351m. When it is cold it idles at 650 and my timing sits at 5BTC with a steady engine speed. However after warming she starts surging between 900 and 1200, timing goes to about 35BTC(the throttle is sitting on the idle screw). I do have a vacuum leak at the base of the carb and i am going to swap out the base plate gaskets, however i just cant see a leak causing the timing to advance like that. I am thinking the advance is causing the high idle speed with the leak contributing to the surging.

The ignition module controls the timing but most bad modules i have dealt with in the past where just dead and i didnt have any spark. So my question is what is causing the timing to advance so far at idle? Is it the vaccum leak, bad module or something else that i am missing(EGR)?

 

Sticky mechanical advance?

 

Try replacing the springs that sit down in the dizzy. It sounds like either they are bad or missing all together.