I have had a Victa Self Propelled Mower with Briggs and Stratton 750IS Electric Start for 5 years with no problems at all - mower used to mow fairly large yard. Oil changed yearly and always fresh petrol used. Quite suddenly, the mower became hard to start ( normally only 1 long press on starter ) and after starting, engine speed changes constantly and within 5 mins it slows and dies. Able to start again after several attempts, but same symptoms again. I have removed spark plug ( looked perfect ), cleaned and checked gap. Re-installed, but still same problem. Also air cleaner filter checked but only blow out required. The various actuators on top of the carby all appear to be free and moving correctly ? Any thoughts on this problem greatly appreciated.
Remove and disassemble the carburetor for a thorough cleaning, ensuring all jets and passages are clear. Check the spark for strength and consistency, especially after 5 minutes of running, just after the engine dies . Perform a compression test. Verify the fuel flow from the tank to the carburetor ,some mowers have a fuel strainer in the bottom of the tank or a fuel filter.
The plastic carbs used on the modern OHV Briggs engines are known to be very sensitive, mainly because they jet them very lean that it doesn't take much to cause the sort of running condition you have described. This applies to the 125, 140, 150 and 163cc engines, I think they use the same carb on the larger-block 190cc engines too. In any case, you will need to remove, disassemble and clean the carb.
These video are quite long, but very informative regarding these carbs -
