Sounds like an ignition issue. Where'd you source your ignition parts? Is the spark bright blue & can the spark jump at least a 1/4" gap? Also- was the ignition coil changed? If so, does the unit have an ignition coil resistor or a coil with a built in resistor? If there's an external resistor- make sure the coil doesn't have a resistor built it. Does the unit still have points or was it converted to electronic with a pertronix unit?
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