A bit of smoke while idling

I have a 2000 GU ZD30. It's always been good with regards to smoke, hardly anything! It's got a reconditioned motor in it with only about 12,000k's. My cat converter was dead so I've knocked out out. I'll be getting a new exhaust soon so it'll do in the mean time.
Since I've knocked it out, I've been getting a small amount of smoke while idling. Occasionally there'll be a bit while under load, but most of the time I don't see any. I find it strange that I only see it while idling.
I'm not sure if it's something else and just a crazy coincidence that it's only started now.
Could it be that the cat takes a bit of that smoke away or is there something else I should look in to?

G'day Booshie,
a generalism I've found holds up pretty well is, if all is good before you fiddle and all is crap after, the fiddle may well have something to do with the new status.
Cat converters are there I think to reduce crap emissions. How they do it is a mystery but I reckon you should wait for the new system before getting to excited.
Coupla reds, settle down and wait and see,
all the best,
Col

Thanks.
It did change the car a lot. I had to open up my needle valve a bit cause it was spooling up a bit quick, and there's a noticeable increase in low down torque.
I was a bit worried about it, but I guess if the car is running better than it was it can't be too bad...

Reckon you're on the right track mate.
Seems to me the less things you change one time the easier it is to keep track.
Luck,
Col