Kohler toilet leave black streaks and small black particles coming out of water holes under toilet bowl rim.

SOURCE: Master bath toilet replaced with a Cimmeron class

Kohler is correct. The most likely cause of this problem is an obstruction in the common vent stack that both toilets share. When you flush the old toilet it need more air to flush and will get it from the easiest source (your new water saver toilet). First adjust the float level in the old toilet so it won't use as much water or just stick a brick in the tank to take up space.Second, run a snake down the vent stack from the roof to check for obstructions. Birds will sometimes nest in there.

SOURCE: Kohler welworth enlongated 2 piece toilet with

Place a few drops of food coloring in the tank of the toilet, if it is leaking you will see the color seep into the toilet bowl within a short period of time. If you see the food color in the toilet bowl you will need to replace the flapper.

SOURCE: water in bowl. water level almost below where bowl

If I understand correctly, your water level inside the bowl is 3 or 4 inches from the top of toilet seat?
Sounds peculiar to have that water level since you would get splashing etc.
That is not sanitary, and would be a code violation.
Look at side profile of tank.
Look at porcelain behind bowl and you can see outline of "s trap."
This is the internal trap that each toilet has built in.
Purpose of trap is to a) set water level inside bowl and b) just like a sink trap, seal pipe from sewer air from entering home.
Photo shows trap on sample toilet, with line showing approximate level of water inside bowl.
Since photo is at an angle, the line appears to go downward instead of level. Again the line is approximation, and photo intended to show where you can look on your toilet.
Your tank should have similar s-trap feature. From the feature on your tank, you can see what level water is intended to be inside bowl.
Some more expensive tanks, outline of s-trap is not apparent.

If the water level is so high that you experience unsanitary conditions, this is a code violation, and toilet should be replaced.

Quick web search turned up Kohler contact page: http://www.us.kohler.com/general/contact.jsp
KOHLER Customer Care at
Phone: 1-800-4KOHLER (1-800-456-4537)
Hours: 8:00 AM - 5:00 PM (Central Time) Monday to Friday
You can refer Kohler to this link on Fixya so they can clearly understand issue.

SOURCE: The bowl will not hold water. This is the second

the tube that goes from the fill valve to the fill tube may not be installed correctly. the water must be going in the tube and not the tank. also it is a water conserving toilet so it uses less water also it is a comfort height toilet and to make it taller the bowl is extended so the water may seem even more shallow.

SOURCE: The water level is too low in my Kohler toilet

check the (white) tube in the tank. it should connect to the top of the black reservoir. mine came loose and had the same problem. you can check the diagram on the inside of the tank lid.

cheers