what does it mean when the timing mark bounces around within a 5 degree range?... like if i try to set it at 12 degrees before, then it flutters around between 10 and 15 degrees... its never steady and when i rev it up, the timing advances, but then it just bouces around between 15 and 20 degrees before TDC.... i'm having a lot of problems getting this thing to run, and i think the fact that i can't stabilize the timing may be at the heart of the problem.... oh yeah, its an '86 D100 with a 225

 

the carburetor could be incorrectly tuned. does the distributor have vacuum advance?

 

The Timing chain may be stretched. How many miles on the motor/truck?