850 I just bought and am tinkering with a 1972 Fiat 850 Spyder.

[DavidHallbuzz]

It will fire and rev up, then dies. The mechanical fuel pump has been bypassed by an electrical one; I'm wondering if that's the issue?

Previous Fiats owned back in the 80's & 90s:
1959 Nuova 500
1970 (or so) 124 Spyder
1969 (or so) 124 Sport
1971 (or so) 128
1972 (or so) 128

 

[bugsymike]

It will fire and rev up, then dies. The mechanical fuel pump has been bypassed by an electrical one; I'm wondering if that's the issue?

Previous Fiats owned back in the 80's & 90s:
1959 Nuova 500
1970 (or so) 124 Spyder
1969 (or so) 124 Sport
1971 (or so) 128
1972 (or so) 128

Put the fuel pipe to carb in bottle and have someone turn ignition only on for a few seconds without cranking, it should soon fill if pump working.
You may want to pull wire off coil for safety/fire risk.
If that is OK check carb needle jet or stuck float etc. clean carb jets.
If engine dies as soon as ignition key released from start position a faulty ignition switch can do it. Fiat 124s and Lada used to have an issue with that, the Ladas when still brand new in showroom.

 

[DavidHallbuzz]

Thank you bugsymike!

So, I sort of got it going. The mechanical fuel pump leaks, the electric one seems sufficient. I pumped out the old gas; it was very yellow. But, new gas is yellow too. I'm guessing that's iron oxide?
Anyway, I determined the choke and cable was stuck. I oiled everything and loosened it all up. I got it running, but only at a fast idle at full choke (choke is on 1 barrel of 2). I drove it around; it ran very poorly and then got worse. It would start, rev up and stall. I got it home on full choke.

 

[bugsymike]

I think you are correct stale petrol/rust in tank and all that going through the fuel system.
So a complete clean through from tank to carb, may even have to replace rusty/repair tank, fuel lines and possibly fit an inline fuel filter.
As an apprentice in the late 1960s the more basic 850 saloons and coupes were quite popular, including the Spanish Seat versions.