My friends really nice, low mile B-20 '69 1800S with a downdraft Weber 32/36 has recently started gassing her out. I'm not a huge fan of the Webers, since they are tiresome ot set up originally, and tend to get weird after a few years of use.
Don't get me wrong, for people who have lost patience, or control, installing a Weber after fussing with OEM carbs, appears to be a magic bullet. Several years ago she drove across the country with this car, and it performed flawlessly. I've recently noticed that there is a lot of blow-by around the throttle shafts,and she tells me that the gas mileage isn't as good as it was (maybe low 20's versus high 20's earlier). I suggested washing and cleaning out the K&N air filter for a start, and possibly a new PCV from the flame trap, plus a new one-way valve from the brake booster. It's had a big tune-up in recent memory, including a valve adjustment, and plug wires. Car has a Pertronix breakerless ignition installed.

Without being on the scene myself (I'm on the Left Coast, she's on the Right), I can only add a few "red-herrings" from her input, and my recollections;
1.)I seem to remember that compression in one of the cylinders (#4?)was lower than the others. Like maybe 120lbs, versus 180lbs. Not sure, but it was more than 10-15 lbs. Maybe broken piston rings?
2.)There was a valve job, with hardened seats in recent memory.
3.)Car gets pretty even local and highway usage
4.)Car gets premium gas almost exclusively

So, what are we looking at here, a new Weber,
a fresh set of SU HS-6's? Reshafting the Weber?

Suggestions welcome,...