Curtis B thankyou but there does not appear to be a flap in the carb.- when the air filter is removed you can see right down into the jet. the engine is primed by pressing a rubber nozzle three times and starts at the first pull. It then runs for about 30 seconds at the correct revs and then goes to about half revs, hunts and at the lowest point emits some black/grey smoke before recovering to the lower revs..
It is difficult to see the identity number as it is at the bottom of the crankcase and partially obscured by the mower body but as far as i can see the identity number is XXXXX Does this help?
here is link to a similar carburetor. It has no choke, you are correct. So it is a dirty carburetor or a bad gasket or jet plugged. Th e numbers you gave don't show up, but the 450 series uses a carburetor that mounts to the fuel tank,--- http://outdoorpowerinfo.com/repairs/briggs_pulsa-prime_carb.asp
thankyou. This is not very helpful as the site does not identify the engine installed in my lawnmower and therefore I am at a loss as to the next step. The answer will therefore be to take it to be rectified by an approved Briggs service agent. All very frustration and a waste of my time, meanwhile the grass continues to grow!