One of the common problems with aluminum block Briggs and etc. engines is that the exhaust valve tends, in time, to sink it's seat into the soft block because it is the hotest place in the engine. [One reason for the rather light valve spring pressure on these engines]. As the clearance between the valve stem and the lifter closes up the poor cylinder filling, lack of adaquate compression, and a confused carburretor results in poor idle, loss of power, hard starting and etc.. A brutal cure that I have used is to pull the cover off the lifter box and remove the keeper on the exhaust valve and pry out the spring. Useing a small grinding stone on a die grinder the stem is assulted and shortened a bit. Rinse out the area as much as possible without getting too much grinding dust in the engine and reassemble. Often the little motor runs fine. My neighbor's 5 HP Briggs on his roto tiller has been running fine for a number of years since that was done. Another problem with an aluminum block is that the flywheel side crank bearing is just aluminum in the common engine. If aluminum ever gets hot it oxidizes and creates aluminum oxide. Aluminum oxide is a popular abrasive for sanding disks and grinding wheels. So what happens is that the steel crank is "cut". That is not a terrorable problem except that the slop can allow oil to get past the seal and very often make the little plastic plunger, that rides against the crank and opens the points, to open the points at the wrong time. Timing and spark intensity is badly compromised. Is there a BRUTAL cure for this? YES! Take off the flywheel and points and useing a blunt punch and a BFH [Big F***ing Hammer] punch the aluminum case around the crank journal. That will usually close up the clearance enough to make the engine [sorta] run normally for some time. A guy that used to buy and sell used lawnmowers showed me these things. Wanta know how he made donuts? Another time maybe.
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