Any idea why a stock 280z 1978 has a back fire problem blowing out blue smoke 

fuel mix not right vacuum leaks, unmetered air, ECU, Sensors, ign timing, smoke from burning  oil from warn out engine, valve guides, seals, rings, pvc issues, wrong oil.

one thing at a time, start with a compression check, then check ign timing, then fuel pressure test. Need a lot more info before a meaning remote diagnosis can begin.

Ditto to all of the above. Some car and engine history would be helpful too..

Try unplugging the cold start valve?

My 240 starts blowing blue when I leave the choke on too long.

Thankyou, will replacing the ingine help get the car starting...won't start or stay on, when it does is back firing

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Plug wires are placed in backward rotation order in the distributor.  It will run, but poorly.

1 hour ago, diego1988 said:

Thankyou, will replacing the ingine help get the car starting...won't start or stay on, when it does is back firing

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I'm usually the one that confuses but you got me with this?

I'm thinking off doing an ingine swap with transmission....guessing this will solve all my problems

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23 minutes ago, diego1988 said:

I'm thinking off doing an ingine swap with transmission....guessing this will solve all my problems

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Well,  that would be one way........  But simply figuring out your current engine would be more economical.  Just my 2 cents.

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I feel like the ingine is dead oil going into the spark plugs and the distributor is Gone. I have almost no mechanic skill

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Owning a Z will turn you into a mechanic.  Very knowledgeable people here that have done it all and then some.  You can start here  http://www.classiczcars.com/files/ by downloading the Factory Service Manual for your 78.

Learning how to repair your Z is invaluable  and easier than an engine swap if you don't have the experience yet.