Thanx Bruce.
I have a Rover with a B&S engine.  It starts on the 2nd pull normally but runs very slowly for the first minute or so.
If I give it too much throttle it stops. After a minute or so I can gently increase the throttle setting to the full position and it runs at full revs with no issues.  There is some old fuel mixed with new fuel in the tank.Is this relevant ..?
regds,
John