So it's started since your tune-up last year, or not? The backfiring was the old symptom or the new? Has the car been run normally since re-jetting the weber?

There's a lot of variables to this, but if the engine's turning over you're probably experiencing an over-rich fuel mixture. Otherwise, you may have some water in the gas tank. Can you see fuel in the pump and filter? Maybe try pulling the plugs and checking their condition, while you're at it disconnect the coil wire to the dist cap (pull it from the coil) and turn the car over a few revolutions and see if your fuel pump is pumping.

If the plugs aren't black, you're getting fuel and you've still got no start, check the ignition system.

Next I'd suggest another tune up, paying attention to the vlave clearances and the ignition advance.

Anyway, this is what I recall from a 70's Chilton manual; which I thought I had here at work but I guess not...

If you narrow it down to the Weber's Jet, to check if the jet is too large (rich) the mixture screw will be less than 1.5 turns out from the stop. It should be 1.5 to 2 turns out. If it's more than 2.5 out, then the jet is too small (lean).

Good Luck, and have a blast. Hopefully it's not raining on you. It only rains when I need to work on the cars... and it's always raining here these days.

-Sean Custer
--
1966 122s; 1970 142s