How do you know it is not vented, it could be stack vented are wet vented etc. Make sure water is up to mark in tank, make sure 1/4′ tube is in 1″ overflow pipe, make sure flapper opens all the way up,check rim of bowl and see if the holes are clean, use coat hanger to clean them out.
In reply to message posted by Retired plbg: How do you know it is not vented, it could be stack vented are wet vented etc….
The point is that the toilet drain MUST be vented. If it’s not, that is a major problem and you need to fix it. It is true that SOME plumbing setups MIGHT conceivably allow an unvented toilet to flush and drain effectively, but such an unvented system is never allowed by the plumbing codes. Please have your system inspected by a licensed plumber.