If your start cord is very hard to pull and the engine runs very loud and hot, your lawnmower needs a valve adjustment! Similarly if the lawnmower has very low compression and will not start, it could also be caused by improper valve timing.
To adjust the valves on a Briggs & Stratton 550EX engine is very easy. The valve clearance is 0.004" to 0.008" for both the intake and exhaust valves.

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