Engine Cuts Out or Idles Low

[gunk]

I am working on an 1800 Engine in a 1983 Hardtop Leone.

It has gas and petrol, usually runs fine on gas but cuts out on petrol if you don't keep the revs up.

The other day we didn't have any gas left so we switched it to petrol and I tried to get it running properly.

I had it idling fine at 850 RPM (+/- 50 as it's a digital dash) and it ran fine all day and night like that.

This morning I start it up again, it's running a little rough with the choke on but once the choke goes in it's idling smoothly at 850 again.

Then after about 15 minutes of driving it's suddenly idling at 450 again and cutting out. So I switch to gas (I put some in last night) and try driving it home like that but same problem.

They COULD be two separate problems (like maybe because the timing has been set for petrol now) but I'm not sure.

Any help would be greatly appreciated.