unstable/sporadic timing with volvo penta distributor

[RubyRed]

Picked up an 87 sunsetter with a blown headgasket. got it all back together and had the first water test today. having some ignition problems. someone previously installed a volvo penta electronic distributor and coil. i noticed when setting initial timing it would periodically jump, very sudden like a light switch. idling around 600 id set it to 10* and it would suddenly jump to 15 or so for a few seconds then drop back. when it would do this it would also make the idle jump 100-200rpm. thought it was strange but went on with with the water test. in the water under load it would not rev past 3100rpm (30mph) and felt a little sluggish out of the hole, no popping or hesitation just slow. at 3100 rpm 3/4 to full throttle makes almost no change in speed/rpm just a slight change in engine tone.

pulled the timing light out and tested it on the water, it was very sporadic and inconsistent. with base timing around 10* i was only getting a max advance of 25* (@3000 rpm) but it was not a consistent 25* at all. one revolution it would be 25, next would be 15, next 21, next 25 etc. i believe it should be 30-34* total advance. i bumped the base timing up to 12 or so and it starts a little easier but made no noticeable difference in power. i know there is a chance its a timing chain issue but being the idle timing changes very on/off like a switch i feel like its not a timing chain issue but rather a distributor issue. 

 

Anyone ever experience this? how much slop should there be in the distributor with the cap off? i have a little bit of rotational slop and up and down slop but doesn't seem like an alarming amount. might just throw a new ignition module in for $50.

Edited May 27, 2022 by RubyRed