Back firing down hill can be a timing issue of the firing point, valve timing (unlikely) and/or the idle mixture being too lean.
Don't pull the distributor off to do the points as the timing should be already set, just clean the points and reset the points and spark plug gaps.
Then the timing could be tweaked if it's off after doing a check.
To do a quick check, align the timing marks to the desired advance for your petrol octane, with the key on, distributor cap off, slightly wiggle sideways the rotor button (don't twist it to do this) and that gentle movement should cause the points to make and break, you should hear the sparking / clicking as a result.
Those Solex carbies can have some fine rubbish build up in the bottom of the fuel galley's and in the seat of the idle mixture screw, there are a couple of brass plugs in the bottom of the carburetor that you can undo to let the petrol flush out most of the rubbish and for the blocked idle mixture, the mixture screw can be tightened in then returned to it's original setting (count the turns), this opens up the orifice and that loosened rubbish can also be flushed out by removing those brass plugs.
One thing to watch out for when the distributor cap is off, inside the distributor there is a flexible earthing wire that is between the moving plate and the inside body, make sure that wire (which looks like a fine braided wire) is in good condition.
You will be on your way in a few minutes.
