I'm running and will be running SU's.

I don't have off-idle stumble but the advance conversion isn't so much for problem solving as for improving the day-to-day life with technology this old. So if vac advance and high energy spark allows for better carb-tuning at cruise then all the better.

In the early seventies I thought the focus was on reducing emissions and lean mixtures and timing tricks did this at the cost of performance. If they were trying to increase fuel efficiency wouldn't power per cubic cc have increased? I was watching Sesame Street when this was happening so I don't really know the history.

I'm afraid I'm not enough of an engine guru to make an informed argument for or against. I'm doing research to see if this vacuum advance is worth trying.

Dyno and gas analysis would probably be the only way to get real info. and I'm not currently planning on doing either soon. But will have to do at some point as seems the way to best tune carb and ignition timing for a truly happy B20.

The basis of a lot of this line of thought is the Classic Motorsports article. Worth reading and should only cost a cup of coffee at the bookstore of your choice, not too long of an article. I mention this not to say you need to learn about ignition, just to let you know where I'm coming from.

As I said I am not enough of an engine guy to be able to make hypotheses about what "should" work.

I am happy to learn what does work and try to understand why.

Best regards,

-Mario E.