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A: Backfiring (and misfiring in general) can be caused by a large number of problems; unfortunately not only you do not even say what kind of Fulvia you have but your description of backfiring is superficial. You certainly have not attempted to exclude some obvious causes of backfiring. With these limitations I can only try to list a number of potential causes and how to solve them.
None of the things you have done (plugs, cables, alternator, filters etc)has anything to do with backfiring. I am not surprised that no improvement was obtained.
You must check that the valve timing is correct and that the timing chain has not jumped one tooth of the camshaft sprockets (details will depend on the car engine). Ensure also that the valve clearance is not badly out.
You must check that the ignition timing is neither too advanced nor too retarded and that the correct dwell angle is set with respect to the points opening gap (details depend on the engine).
It is unlikely that the problem is due to carburettor malfunction: however, check that the two carb floats are OK if everything else seems fine. I presume that by inspecting the spark plugs you concluded that the carb air/fuel mixture was basically correct.
A leak in the intake system can cause backfiring but you do not even say if your car has a brake servo (some Fulvias did not have one). It is very unlikely that the servo is causing your problem since you would notice that a strong effort is needed to stop the car and that the same amount of brake pedal pressure can be applied with the engine running or switched off. It is possible, that the vacuuum hose running from the cylinder head to the servo (or its one-way valve) is damaged: a crack in the vacuum hose occurs very often although I have not seen it to produce symptoms as bad as the ones you describe. Any vacuum hose (with suitably rigid wall) of the same diameter will be OK to replace yours: it does not have to be a Lancia product.
Your car may have a leak either in the carburettor rubber support or in the intake manifold gasket.
I do not know if the Polonez servo can fit your Fulvia. Reconditioned servos can be obtained from commercial suppliers but I suspect that the servo itself is not the cause of your problem.
Poor throttle response (when everything is tuned up correctly) can be due to failure of the centrifugal advance mechanism in the distributor (broken springs, stuck bob-weights etc) or to inefficient acceleration pumps (holed diaphragm) at the bottom of the carbs. However, neither of these problems should create backfiring.
Fulvias do not necessarily start instantly from cold because, if the car has not been used for some time, the carbs must be primed, a process which takes a bit of time as the fuel pump works while the engine is spinning. Some people have fitted an electric fuel pump to prime the carbs. Of course, using the choke will help starting from cold.
Finally, it could well be that, after you have identified and cured the problem, you must balance and tune the carbs. It is easy to blame carburettors for all sorts of misfiring problems: in practice, they are very robust and long lasting but they should be balanced properly by measuring simultaneously the vacuum created by each carb pair at the appropriate plug hole in the intake manifold.
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timing issue. Remove the flywheel to see if it has "spun" from the keyway. If the motor doesnt run at all but just backfires thru the carb, it could also mean really bad intake valves, but I would definitely check the timing first