no power, but burns lots of gas
 

My wife's 1990 Mercedes 300E has very little power throughout the RPM range, starts well when it's cold but is hard to start when it's hot, it use to get 19 MPG now it gets 10mpg. The spark plugs are burning hot and are ash white.
I guessed that with ash white plug I was running too lean so I started with the fuel filter and replaced it. No change. The engine pulls 15lbs of vacuum at 700 RPM. The rotor, cap and plugs were also replaced.
What should I check next. And how do you time this car.
I own a 1971 Mustang 429 CID which is child's play to work on compaired to this Mercedes.

Have you considered an exaust restriction?
10mpg sounds a little extreem for a fuel mixture problem... but you could take a look at the duty cycle with a milti-meter: http://www.landiss.com/mixture.htm