Thankyou Machinery pro for your helpful answer. The Mountfield mower is an LS45 with a Briggs and Stratton engine. I have cleaned the air filter, soaked in petrol and dried. The plug is clean and reset. Should I replace it? I have WD 40'd all the loose moving levers and springs. The pulsing action and small puffs of accompanying black smoke still persists. Running rich? Do you think it is the diaphragm and if so when are you publishing the tutorial and where can I get a replacement?
Following your advice I changed the diaphragm/gaskets between the carburettor and the fuel tank on my Mountfield LS45 lawn mower and it now works beautifully. Thank you. It no longer pulses and puffs black smoke
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Try fixfault.com to fix lawn mowers.