Hi. I am having trouble with three of my 9 lawnmowers. Two of them will start on half throttle, but will cut out if you go to full throttle unless you let them warm up first. They are both briggs and stratton 3.5hp engines. One has a upward pull start and the other has side pull start, but are practically both the same. I will try and find out models, but one of the engines has been re-painted. The 3rd mower, the engine has been on the patio rusting for the last 6 years. I put a new spark plug in, added a fuel line (it didn't have one) and tried starting. It eventually starts after about a two minutes pulling the start. It then will go to full throttle fine, but cuts out after about 5 minutes. When it cuts out, it goes in a plume of smoke, smoke from just about every place on the engine (I suppose that is 6 yrs of rubbish burning off. It doesn't have too much oil either. It also seemed to misfire a bit, when it is about to stop, and knocked the air filter cap off and the filter out. It then smoked from there. This one is a Tecumseh. I have checked the spark plug gap, which is 28 (7.5mm?)
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