My problem toilet sends air bubbles out of the siphon jet opening when it is flushed. I get bubbles first and then water. The bubbles disrupt the flushing action and slow it way down. The slowness of the flush seems to encourage clogs. To get a halfway decent flush I have to hold the lever down until the water actually starts down. The tank is filling to the fill line. My other toilet, that doesn't have a jet, flushes just fine. The one with the problem is located closer to the street sewer line.

Has this been a problem from the startor did it come on suddenly?
Are they big or little bubbles? The bubbles come from air trapped in the rim, (see image) .
If I had to guess it would be that some of the rim holes are clogged and not allowing all the air to vent from the rim.
Check the rim holes and if you find any that are clogged clean them out with a stiff wire or coathanger. Good luck, Tom