My fiat 850 spider tachometer randomly begins to waver and the engine begins to run rough. It starts easily, dwell is spot on at 60 deg and timing is correct.
Battery has plenty of juice. Regulator is new. Coil is new. Distributor points, cond., etc... all new. Any ideas to help correct would be appreciated. Thanks!

Sounds like something is disrupting the regular sparking of the ignition system thereby causing erratic running and wavering tachometer (iirc this works by counting ignition pulses).

Maybe it would be worth checking out the plugs HT leads, then distributor cap and rotor arm, then replace condenser. (not sure if these have already been replaced). A failing ignition condenser sometimes works fine when cold but acts up as engine gets hot. Pay particular attention to the low tension wire that goes from the coil to the distributor (iirc green wire). I've sometimes found this wire to be chafed and intermittently shorting as engine rocks or alternatively, loose connections at either end, especially at the distributor. I've also seen the wire/cable loose in the crimped on terminal.

You could also try running a direct feed from battery to +ve (B+) on ignition coil to rule out any intermittent connection to the coil (I'm thinking possibly faulty ignition switch, either internal contacts or connections at rear).

Al.

67fiatspider said:

My fiat 850 spider tachometer randomly begins to waver and the engine begins to run rough. It starts easily, dwell is spot on at 60 deg and timing is correct.
Battery has plenty of juice. Regulator is new. Coil is new. Distributor points, cond., etc... all new. Any ideas to help correct would be appreciated. Thanks!

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Were all the new parts fitted before the fault appeared, or to try and cure it?
If fitted before, you could have a faulty part. spark plugs and "condenser" (across the points) are common causes.


Robert G8RPI.

If fitting new parts, do fit a new distributor cap. This can cause this, as any scratches inside will cause arking. New plug leads too, as they do break down. Try running engine in the dark at idle. This can show any sparks escaping.
But one of the main problems, is the low tension lead. This can cause it, as the lead will heat up, then start to break down.
Another possibility, is the coil. Again, a faulty one will heat up and the windings, break down. A good test, is to touch the coil after a good run. If it's hot, it's failing.