Not exact matches
What's mostly to blame,
on modern cars at least, are systems that ignite
unburnt fuel in the exhaust systems to try and warm up the catalytic converters as quickly as possible.
With this loss of energy, you'll also see the large carbon buildup
on the business end, due to
unburnt fuel deposits.
Valves rotate slowly to scrape off the
unburnt fuel particles settled
on the valve seat.
Change down a few gears, and you'll hear some phlegmy overrun as it spits
unburnt fuel out of the exhaust, let it hit the rev limiter and it'll bounce
on it frustratingly.