Hi Ollie,
Surging is a sudden change of the fuel and air mixture during each complete cycle. Each intake stroke will sucks the mixture into the cylinder. Anything after the air filter will affect the control or governor and mixture ratio into the combustion. What factors that affect this? With newer engine it's unlikely to be the governor spring since its spring tension won't deteriorate that quick unless rusted or someone had stuffed it. Air leak on the manifold oring could affect the surging. Partial fuel blockage in the carburetor jet is one of the main problem. I don't recommend to oil the foam filter as this will attract dust and dirt that will restrict the air flow and makes the mower hard to start. Partially restricted air filter won't affect much on surging but will affect on revs and iddle. I have serviced 500e and 500ex but I can't seem to find a fuel filter in it. Could be inside the fuel tank but I haven't check it. Why I wondered because why everytime I rebuild this plastic carby there's always a dirt in the bowl and jet and how the dirt got in if the fuel is filtered? This type of carby doesn't have mixture adjustment. With the Classic/Sprint engine, surging is due to air leak through the diaphragm between the carby housing and fuel tank. Anyway I need an ignition coil for Briggs 500ex if anyone can sell. Please contact me.
Cheers,
TheCarbyMaster
