Hi,
Yorkies are known to have early tooth decay. You will have to brush their teeth at least once a week. My yorkie had the same problem. I have a plan with my vet where I can get his teeth cleaned once a year. This is definatly a must to keep his teeth in good condition.
Are you feeding alot of wet food? This can give them really bad breath as well. The dry food helps to keep their teeth clean. You also can give them toys that help clean the teeth. My baby like the dental bones and he loves Ropes. The constant chewing of ropes and hard toys helps to keep their teeth clean.
Before I brush my babies teeth I put some toothpaste on the toothbrush and let him lick it off.
You know how humans clean their tongue.
I have found that doing all of these things has definatly improved my yorkies breath. Actually I don't even smell his breath anymore.
Good Luck!