GEO PRIZM O'2 sensor

Hello

I have a 1995 GEO PRIZM - I had the top half of the engine redone(head gaskit kit,plugs,wires,dist etc)

Drives well but when you rev the engine alittle while in park-idle it spits out black smoke- Is this sympton due to a O'2 sensor if so I have 2 of them and they are not cheap. I was told I should replace the front one by the manifold first before getting the other one. What do you guys think?

Thanks

Mike

very possible definatly if it smells a lil rich.it can be oil getting in the combustion cycle i first recommend clean the engine with seafoam an see what happens