Lawnmower is spewing white smoke

The white smoke points to the oil and fouled spark plugs.
Make sure the oil level is according to the gauge.

Most likely blown head gasket or engine crankcase overfill with oil/gas from an leaky carburetor. Note once oil get into the muffler it will take a few minutes for it to burn off.

check the oil level see if its overfilled and smalls like gas if so the needle valve in the carb is not seating replace the needle and seat add a **** off to the fuel line and change the oil gas leaks passed the needle valve overfills the carb and drains into the cylinder and crank case run the carb out of gas by shutting off the fuel line and run it til it stalls out every time your done using it

There are few possibilities that can cause such problem.
I recommend you few help links related to your problem.Click the link below and follow the troubleshooting;-----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------
Brake pedal stuck? http://howtobyme.blogspot.in/2012/03/brake-pedal-stuck.html

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Car only starts when gas pedal is pressed? http://technoanswers.blogspot.com/2012/01/car-only-starts-when-gas-pedal-is.html

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Car won't start without hitting the gas pedal? http://schematicsdiagram.blogspot.com/2011/12/car-wont-start-without-hitting-gas.html

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emits whitish bluish smoke and bad fuel smell from exhaust

http://schematicsdiagram.blogspot.com/2011/12/van-emits-whitish-bluish-smoke-and-bad.html

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White smoke and Acrid Smell? http://whoisbyme.blogspot.in/2012/07/white-smoke-and-acrid-smell.html
---------- Car stalls and throws white smoke from exhaust? http://repairhelpcenter.blogspot.com/2012/01/car-stalls-and-throws-white-smoke-from.html

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This will help.
Thanks.

head gasket is bad

usually large clouds of smoke are from having too much oil in the crankcase, or tipping the mower so that oil runs into the cylinder. Check the level of oil, and make sure it's not overfilled.

Spews smoke and then stops? Worn valve guides or worn valve guide seals. Quick way to tell: take it for a ride with someone on another bike / car behind you. Use engine braking. Puff of smoke? Probably seals.
Bright side: These engines are dead easy to work on.

Fix your car right away, by clicking the link I attached
JAGUAR XK8 1996 2006 WORKSHOP REPAIR SERVICE MANUAL

Clean Carb and adjust

The Symptoms tell me it is a head gasket. A leaky or bad head gasket would cause all of the problems that you mentioned.

Burning water causes White Smoke, Burning Oil causes Black Smoke. The oil and water are mixing with the gas in the cylinders and causing it to run bad. Pull out the dipstick and look at the oil on it. Does it look like good oil? Or does it look watery and or foamy or grey colored. Open up the radiator and look inside, do you see oil?

This seems almost text book to me... I would have someone local to you to have a look at it to be sure.