62 ishing scott, carb

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62 fishing scott mfg by McCulloch I have a vent tube on top of my carb and the hose that connects to it has this hard metal bulge at the bottom. Is this so gas does not enter into the housing ? Also, I took the carb apart and everything is new, The diaphram reed valves and gasket, the reed valves are laying flat like their suppose to. also new coils points condensers and spark plugs. the cylinders are getting gas and I have new spark plugs. Any reseon why this would't even give a hint of starting ? vent on gas tank open and primer bulb works good. It back fired a couple of times. No sheared key. Thanks