IIRC, you went from 10 degrees to 15 degrees initial timing and stopped there? To truly know where the "sweet spot" is, you must go too far, then back off (approach "dead nuts" from both directions so you'll know you're there). Bump the timing up some more until it quits producing more idle vacuum/smoother idle/better throttle response or until the hot engine kicks back against the starter when you try to restart it. Then and only then will you know the initial timing is optimized. My guess is 18 - 20 degrees will be about right, but there's no way to know without trying it.

After that you'll have to disassemble the distributor and (not JB ) weld up the advance slots and file them to the correct length to get the total advance back down to 36-38 degrees. If you haven't already done so, you might want to think anout recurving the distributor with a lighter spring at that point. The engine is in a Jensen, and they're fairly light cars, yes?

If you're interested in using the vacuum advance, you'll work on that next. Tuning it consists of adjusting the level of vacuum required to actuate the vacuum advance mechanism (easy) and possibly limiting the degrees of vacuum advance, based on what your engine will tolerate (not quite as easy). You'll find that the opinions of Moparts members regarding vacuum advance varies widely, with some swearing by it and some swearing at it.

Assuming the rest of the ignition system passes muster, then it's back to the carb.

Post back with your results.

Jim