My two toilets are flushing okay, but I can see 40-50 small bubbles rise to the surface with each flush.

Is this normal? I know that partially clogged air vents can often cause such bubbles, but at what point is a plumber call needed?

If I may tag along with your post, I have a similar but more pronounced problem. My three toilets at times need 2-3 flushes to clear waste; and one in particular does a sizeable bubble eruption at the start of a flush. Water level settles very low.

larryw.. check the water level in the tank.

I also get that "belch" at the start of a flush sometimes.

Bubbles in toilet flush can be caused by improper vent size or partial vent blockage. When you flush, the air that enters the toilet trap has to go somewhere and if the vent is clogged or blocked, bubbles will come back up through the trap into the interior of the building. I would thoroughly check the vents and also make sure the drain is clear. Run a camera down inside the pipe to know what is happening and where the problem is.

If vent blockage can be a problem, wouldn't it make sense to put a screen over the top of each vent? I don't believe I've ever seen that done.

I clamped on wire mesh on top of the two vent stacks in my house to ensure nothing gets dropped into the pipes.

Packy - thanks. Water level is at the manufacturer scribed fill line.