Strange ignition problem

i've got an ignition problem thats had me stumped, maybe someone out there has an idea of what's wrong. my engine (250 ci ford inline 6 w/ stock points) has been missing pretty badly. I timed it and changed the plugs a month or two ago and it had been running like a champ. I threw the timing light back on it and the mark was still at 12 degrees BTDC, but every time it missed it would drop down to like 7 degrees and then back up to 12. this drop is still consistent at higher rpms when the advance kicks in. i replaced the coil, but the problem persisted. i took off the dizzy cap, points and condenser look fine, cap and rotor look clean (cant see any carbon tracking).
does it sound like a short in the electrical system, or maybe something in the distributor? It's probably something obvious, but i can't seem to figure it out. thanks.

 

Does it have vacuum advance?
Maybe a vacuum leak?

 

hermosa1: You should PM GMC BUBBA he is the Dizzy master..

 

When the advance moves maybe opening a wire on the points, or you have a bad condenser,

 

Use your timing light on each plug wire to find the one missing. Trace it back from there. If they are all firing, then check to see if your vacuum advance is working correctly by plugging it off.
That's just to get ya going on the path to figuring it out.
Do you have a vacuum gauge?

 

Check for bad plug wires. They will drive you nuts, if they are borderline and only miss intermittently. The best way is to pull them off and check resistance, or use a tester that reads good/bad. I've also used a cylinder balance check, shorting one plug at a time to see of I get equal rpm drops. If your getting an intermittent miss this may not catch it.

New wires are no guarantee either. The one I chased the furthest had brand new wires on it. Ran fine before I tuned it. Only missed occasionally at idle in drive, just as you gave it pedal. That's the one that taught me you can't always trust new parts.

 

i like using a vacum gauge to help diag these kinds of problems. it can help detrermine if you have a valve sticking. the timing dropping during the miss is normal, its a result of the miss. check the plugs, see if one looks different from the rest. the timing light on each wire is also a good suggestion. i have even used a test light connected to ground and run the tip of the probe along the ign wires including the coil wire and coil. if the wires are leaking you will see the spark go to the tip. the test light wont illuminate but provides a path for the spark without lighting you up. getting the wires damp helps to.

 

How do you know it's ignition? Maybe bad gas or a little dirt in the carb? Lippy

 

How about being so old that the slider plate and weights for the distributor mechanical advance are not moving smoothly and hang up every so often. Pull the distributor apart and lube the pivot points.

 

Good one Lippy. That's where I was going with this one. Just wanted verification that the ignition is OK.