RPM will surge/Pulse from 700 to 1400

When starting from cold, the cold start system such as auto choke, thermal valves etc would be working/opened. This would increase the engine rpm and keep it running well.

As the engine warms up, the auto choke would beopened fully, thermo vacuum valves would either open ofr close depending on what component it is linked to. Under such conditions, the engine rpm would be reduced to the adjusted idling speed.

If any thermo valve is not operating correctly or the component it operates is bad/has vacuum leaks, the idling speed would be affected and can run erratically or even stall.

I would suggest checking for leaking vacuum lines for a start. Ensure the fule cut solenoid is working correctly as it would cause stalling if it is not woring.