[78XC]

My guess naturally since it is deiseling on would be that your ignition timing is out by a just a fraction to effect running. (Naturally I'm asuming that you have an Electronic distributor and that you won't be having any trouble with point gaps). The reason that it keeps running is because since the spark is getting delivered at the wrong time (being carby I assume?) you are it still drawing atomised fuel into the combustion chamer and its still getting a little spark to ignite that fuel.
Personally I would get a timing light and put a decent chalk mark on the timing markers, un-do the bolt that hold the dizzy in place and adjust your timing (retard or advance) till both those chalk marks line up.
If you need to know how to adjust timing in more detail just say and I will point yo in the right direction as best that I can. I'm not claiming to be a machinic thats just what I have been taught.
Thanks,
Andrew