when the coil fires you need the rotor blade tip to be fairly close to the cap terminal cuz if that distance is too great the spark cannot jump it repeatedly & it will act up which is seen as a misfire and only the can alters RP. the can is causing the problem (one way or the other) so with it on manifold it may cause a problem at idle (just too much timing) but I was thinking it was above idle. the can may or may not be shifting RP to where there is a problem. rarer is a leak in the can diaphram causing a lean misfire. As Stump said get your timing to a reasonable baseline & I would highly suggest plugging the can into ported & see what develops & holler back. EDIT Are you saying you dont have a dialback? (1) degree is 0.0632683" around the dampener circumference and you can cut/measure a strip of newspaper with your calipers & make black ink marks on it to use it as a ruler to make new white paint marks on your dampener, IE 17.5/20/22.5/25/27.5/30/32.5/35 degrees. for example 17.5 deg is 1.107" (CW from the OE TDC slit) MORE EDIT I reread & it sputters on ported or manifold. I'd cap the can & set initial at 15 for a start then plug the can into ported (I'm assuming your idle speed is low enough so there is no vac at your idle speed at the ported nipple) & try it