Remove vacuum line from carburetor and apply vacuum to the pipe with a Mighty Vac to see if it holds vacuum.
You can use your mouth to draw vacuum and then plug the end with your tongue.
It should hold vacuum on your tongue until you are sure it is holding or until you hear someone coming to see why you are sucking on your truck.
If it holds vacuum, push/pull the flat metal actuator which goes into the vacuum advance to make sure it is free or bound up.
Carl
