how do I stop engine run on?

Now and then I get engine run on for a second or two after I shut down. It's really not a big deal, but I'm wondering what causes it, and what adjustment I need to stop it. Thanks for your help.

My engine:
360 bored 30 over with stock pistons
low cmpression, probably 8 or less
360 heads mildly ported and milled .015
summit cam 480 lift
eddy performer intake
thermoquad carb
headers
orange box elect ignition

I run pump gas 89 octane.