A novice's question

I owned my excavator not too long ago and am a novice. When I start my excavator it emits black smoke. why is that?

Well, a lot of stuff can cause that. Depending on the machine, it may be perfectly normal. On most machines, if it clears in a few seconds, don't worry about it. The place to start is always, what machine is it?
A serial number is usually helpful if we have to get in to it.

Make, model and serial number are essential before we can give any sort of advice.

If it boils black at start up and clears up quickly, that’s normal.
The inj pump internals move to “starting fuel” which is about 3x the amount it burns normally, just to start the engine.. then moves to normal run position..
If it keeps black smoking, pull the air cleaner’s and rev the engine a few times and inspect the smoke..
If it’s better but not gone..
my next move would be to have the injectors checked..
Depending on what part of the country your in and where u HAVE TO take them to, save your pennies.. Some fuel shops will charge u just to check them.!!
They tell you you owe them 125.00 and oh, by the way, ALL of them are bad.. We can fix them for another 650.00..
A turbo can be a problem too, but in my opinion, the injectors should be looked at before condemning the turbo..

Thanks you all! My machine is CAT 312C. And it works normally now. It looks like my oil isn't burning enough