How likely is transmitting HIV through oral sex with a canker sore present?

There is a chance. Hiv is a blood-borne disease, so any open sores or breaks in the skin/gums can transmit fluids that carry the virus.
Bad idea. If the other person has previous been exposed to the same virus then they should be immune, however sores are an active out break of a viral infection and contact with others to the sore in any form should be avoided.

Related Questions

Well here goes.... so about a month and a half ago I gave oral sex with a canker sore in my mouth to a man I didn't know.... he said hes clean of hiv?

Risky Behavior. Giving oral sex with an active oral lesion is not advisable for you or your partner. Both of you should be checked out for any possibilities of disease transmission that you are unaware of. Your partner's statement that he is not HIV positive doesn't cover the gamut of problems you could both experience. See your physician and urge him to do the same. Herpes is easily transmitted! Read more...

I bit my gumline and got a canker sore (not outbreak). I gave oral sex. What are the chances of transmitting hsv1?

Use protection. Not because of the recent outbreak, but for your general protection. Hsv-1 & 2 shed even when there is no visible outbreak and this is when most transmissions occur. Use protection regardless. Read more...
Virus vs canker sore. The answer depends upon your viral status. Herpes is a virus, canker sores are not. If you have herpes you should not kiss or have oral sex, nor should you share cups, spoons, etc. It is a communicable disease. Canker sores are known as aphthous ulcers. They are probably autoimmune disorders, but are not spreadable. You need to see a knowledgeable dentist to find out which sores you have. Read more...