90 cadillac eldorado-maybe electrical not sure

i have a cadillac eldorado 90 model i have this problem that when i start it, it floods or dies on me and blows out black smoke i had a diagnostic put on it and they told me there was not much he could tell me except if i
took it to the cadillac house they might could tell me more.I need help to know what the problem is it drives sometimes and when it doesnt drive the way its suppose to it tries to die or it blows out black smoke. also the speedometer doesnt work half the time. Does anyone know if its my cadillac converters that the problem is or what.

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