I have an f-250 with a 390, the sticker on the valve cover says the timing should be 16 degrees BTDC. When I set it there it shakes very bad (like it was terribly out of balance) when I retard the timing it runs fine. Why wont it work with the factory setings? Its all stock.

 

Could be the harmonic balancer has slipped thus making the timing marks inaccurate. In this case tune it where it runs best and still starts ok when hot. Good luck, ray

 

The centrifugal or vacuum advance units may be hung up in an advanced position already.Verify that they swing back into place after pushing/pulling on them.Of coarse,the vacuum has to be unplugged while checking timing!
Phillip

 

My '84 F-150, 302-2V, did not run well with the timing set to stock specs and got 11 mpg. After advancing the timing about 15 degrees, it has more power and gets 14 mpg.
As far as I can tell, the damper ring has slipped on the hub, thus giving false timing readings.