Random mis at idle, gets worse at speed

[SSTKO81]

Over the last year or two I’ve noticed a random mis at idle. It doesn’t usually start happening until the engine warms up, but then will happen every 2-5 seconds. As I idle up, it will become more pronounced. But if I get about 2500 RPM it becomes terrible to the point the tachometer bounces all over the place. - this is on a 1981 Mercruiser 140. Ignition system was upgraded away from points to a voyager a few years back and worked great initially.

 

[Bt Doctur]

could be from a bad manifold or riser leaking into a cylinder