The chopping sound electrical to me.
Make sure your spark plug boot is on all the way, and make sure it is not corroded, cracked, etc. Also, make sure the actual plug wire is in good shape, that it isn't nicked, or falling off the external coil or inside the plug.
Also, make sure your points are clean.
I'd say a good jet size for that kit + stock pipe and carb is going to be in the mid/high 60's or low 70's.