I have a 78 f250 w/ a stock 400 motor. It runs great for having 130,000 miles on it, and doesn't really smoke, but for some reason it idles very rough, and will die if I don't give it gas. When I give it gas, it will idle fine for maybe 10 seconds or so, then get progressively worse until it dies. I noticed that the headlights dim slightly as it is doing this, but I think maybe that is normal ? The alternator was tested fine a day ago, and it has a new voltage regulator. Any ideas as to what it could be ? Perhaps a fuel delivery problem or vacuum problem?

when it idles, it revs up, then almost dies, then rev's up again, and almost dies, and keeps doing this until it finally dies...hard to explain, sorry

Thanks for any help. Let me know if more info is needed!

Mike

 

I would check all your vacuum lines.

 

It also seems to only idle rough when the engine has warmed up.

 

Vacuum leak under or in the carb.