HEI ignition timing leaping off a cliff

71 Jeep CJ5, freshly rebuilt Buick 225 V6, HEI ignition, Hedman headers
Location: 7400 ft

Just rebuilt the engine & carb (rejetted to .048", added brass float) and trying to go through the tuning process. I'm following the idle screw [2-4.5 out]/idle cam/verify timing procedure but we're blown away by an issue that's occurred several times. The timing will randomly jump ~20 degrees without the housing rotating. Vacuum advance is disconnected for the idle tuning. Initially started at 8* BTDC but found 13* to run best so far. Got the idle to a get within 13-15" and the timing jumped to 12-15* ATDC!!!! Reset it back to the 13* BTDC. Did some more tuning, jumped up to 20* BTDC & fell to 4* BTDC while we were watching it.

WTH could it be? Sticky mechanical advance system? Poorly seated cap? Bent rotor tip?

Thanks for any help!

 

Check and see if the mechanical advance is sticky or worn, check the timing chain tensioners, and check the distributor drive gear and see if the roll pin has sheared. I've seen em sheared but the gear was tight enough to not spin but would periodically slip in the shaft

 

Timing mark jumping about 20* so checked damper pully rotation and too much slop. Replaced gears and chain. Still getting jumpy timing. Removed dist and found roll pin almost worn in two (replaced) and lubed the weights. Still had jumpy timing. Replaced advanced springs and timing is now solid at idle and a smooth advance at slow rev up. Runs good.

 

Check to make sure your timing lead isn't too close to another wire. Both of my timing lights will pick up or sense a spark several inches away and trigger the light.

I have to isolate the #1 wire when I'm doing it. Otherwise I can get a false trigger from a wire other than #1 and it shows the timing differently.

Just a thought.