How much carbon residue on muffler is acceptable?

[RedRide]

Originally Posted by Yaris Hilton

When you stomp on the gas pedal and exceed around 70-80% of maximum engine load, the ECU goes into open loop mode and enriches the mixture, commonly enough to soot up the pipe. My Yaris never blackens the chrome tip during cold engine warmup, but does if I go to wide open throttle.

Yes, a little soot on the inside it the tail pipe is completerly normal if you don't drive like a '99 year old granny.
Personally, I always drive a bit agressively and have always had some soot in the tail pipes on all my cars.

Also, if you live is a colder area (or where there are cold winters) the engine runs richer for a short while when first started in cold weather.