Sounds like a fuel issue. Maybe a clogged fuel filter. blocked jet/jets in carby. Thats why when you open the throttle there is no fuel there and then stalls. So you have to open slowly as too much air  and not enough fuel blows the fire out.

 do not let it idle for that long slow idling can stuff the crank up + idling with out a cooling fan on the motor is bad , but anyway replace the inlet manifold seals + clean the fuel; system complete tank fuel cock carb jets   etc min