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Check these: 1.) Black smoke when it does this? -- Air filter dirty/air intake restricted. -- Bad carb. Thoroughly clean and rebuild it, adjust it up right. 2.) No thick black smoke? It could be: -- Loose/dirty wire connection in ignition. Seen them cut out when vibration from higher RPM makes them wobble. -- Bad plug wire (see above). -- Dirt/oil on points, or on block where points are mounted. -- Sticking/worn point actuator rod. -- Clogged fuel tank cap vent, fuel line, fuel filter, carb inlet, float needle/seat. -- Blown head gasket...head gasket leakage...look for areas of oil/gas around outside of head from stuff blowing past gasket. If suspect, pull head and look at gasket for black areas, burned areas. If you have this problem, use wet or dry sandpaper on flat surface and rub head against it to true up gasket surface on head before reinstalling. -- Sticky/carboned up valves -- Usually the exhaust valve that does this. Sure sign is, it seems to run better when cold, then problem gets worse as it warms up. Pull valves and lap them in with compound. Reset lash.
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