Thanks Colin. When there isn't a load on the engine (in park and idling) and I check the timing marks when I accelerate the engine, the advance appears to be working but I'm going to check it anyway. Can I check the function of the counterweights without dismantling the distributor and, if not, what are the tear-down steps (Haynes has a lousy diagram)? As far as the Webers go (and Dan, you'd probably have an accurate price quote for the whole job), I thought the swap would be quite a bit more expensive - I'll check into it! Thanks for your input on this!
Larry and Dan, I spoke with my local MB "parts guy" and he believes that depending on where the leak is, it could be the intake/exhaust manifold, even if the idle is smooth. She has absolutley no second thoughts about backfiring now - very bad. Regardless, I'd sure still like to solve the Solex/Zenith dilemma - I have the carb numbers and am going to search in these archives and the web for some info. Same parts guy said the carb numbers should start with "ZE" and they don't, so I'm not so sure these are Zeniths afterall.