Weird Timing Jump

[200two]

I’ve been doing a tune up on my 74. All the basics plus recently did a carb rebuild (for my 38 Weber),  changed my jetting and put in a new float. Now doesn’t smell of fuel anymore and runs pretty decently.

 

Did the valve adjustment, new spark plugs since others were completely black (they’re still very clean after driving about 300miles so seems my carb adjustment was a success), changed the valve cover gasket, distributor gasket, distributor o ring to get rid of some oil leaks.
 

I set the timing a bit ago but since I changed the distributor O-ring and pulled the dizzy, I wanted to set it fresh. I confirmed my marking is TDC when I did the valve adjustment, but when I go to set my timing the mark is jumping all over the place. I attached a video below.
 

I’m trying to track down a rough idle as well as this timing jump issue. Has anyone had this issue before? My novice brain is telling me it may be a vacuum leak causing both. About 80% of the time, the car shakes pretty decently at idle but very smooth while moving. 

Edited Wednesday at 01:37 AM by 200two

[Son of Marty]

Try it on another cyl and see if the jumping is still there if so It's something funky in the dist, cap or rotor, if it's steady you most likely have a bad sparkplug wire or plug, which plugs did you use as a replacement?

[200two]

23 minutes ago, Son of Marty said:

Try it on another cyl and see if the jumping is still there if so It's something funky in the dist, cap or rotor, if it's steady you most likely have a bad sparkplug wire or plug, which plugs did you use as a replacement?

 

I put NGK BP5ES plugs. I’ll try to check timing on another plug this weekend like you suggest. Some of the other threads mentioned some play in the distributor could cause timing weirdness, but not sure if it would cause this jumping issue?

[John76]

10 minutes ago, 200two said:

play in the distributor could cause timing weirdness,

Yes it could. 

When you had the dizzy out to replace the O-ring, did you check the axial play of the shaft?

Should be about 0.005" play between the gear and the dizzy shaft. Cause for excessive play is the deterioration of the fiber washer. (see attached pic.)

Too much play can cause the timing mark to bounce around as shown in your video.

 

[Buckeye]

+1^

Time for dizzy refresh. I had similar problem several years ago and missing fiber washer caused timing to jump like yours.

 

We have member and dear friend by name Mr. Tom @'76mintgrün'02 that possibly can help you out because he knows these dizzys in and out and got equipment to test them too.

 

 

[200two]

Thanks all, this is very helpful. I did not check for axial play but I’ll get it back out and investigate now that I know.  Will try to reach out to Mr. Tom as well. Will keep this thread updated once I get to the bottom of it

Edited 23 hours ago by 200two

[Dudeland]

Check to see if your rotor is tight on the shaft.  Bad manufacturing may cause the tolerances to be out.  It seems like something is flopping back and forth.   Do you have mechanical or vac advance?