The man's bike was struck by a lorry at 17:15, near a petrol station north of the roundabout junction with the A830.
They warned drivers to expect delays in the area as there are no local diversions.
Officers said at 21:00 on Thursday they expected the road to be closed "for some time" while the emergency services dealt with the aftermath.

While being a fan of "Scandi dramas" like Borgen, Theresa May told the Radio Times she'd tune in to Doctor Who and "a nice Agatha Christie".
Earlier in the day, she'll have a drink with friends before helping churches in her Berkshire constituency who provide lunch for elderly people.
The PM also said she enjoys Strictly Come Dancing, but wouldn't take part.
"I always like to see Doctor Who on Christmas night, if possible, and a nice Agatha Christie to curl up with. David Suchet was a great Poirot - he got him to a T," she said in the Christmas issue of the magazine.
Church has always been an important part of Christmas, she said, as her father was a vicar.
"As a child I had to wait until my father had finished his services before I could open my presents. It felt like a very long wait."
Theresa May: The woman behind No 10
Discussing her TV tastes as a younger woman, the prime minister said she liked watching Diana Rigg and later Joanna Lumley in The Avengers, but never viewed them as role models.
"I have never had a female role model - I've always just got on with doing what I am doing," she said.
Mrs May admitted she had not seen much of former shadow chancellor Ed Balls on Strictly as she tended to be busy on Saturday evenings.
Asked if one day she would be tempted to give the show a try herself, she replied: "I can't dance. It's not a good idea."

Governor Rick Scott issued the executive order in four counties after samples taken from the St Lucie River tested positive for toxins.
The green slime appeared when polluted water was discharged from Lake Okeechobee to prevent flooding.
The toxic algae can be harmful to plants, animals and humans.
The US Army Corps of Engineers began discharging freshwater from the lake more than two weeks ago.
Lake Okeechobee, the largest freshwater body in the state, is contaminated with runoff that includes human waste, animal feed and fertiliser.
The process sent millions of gallons of the nutrient-heavy water into the river and triggered the growth of blue-green algae on some of Florida's southern rivers and beaches.
Samples taken in the river near Palm City and Stuart tested positive for hazardous levels of toxins.
If ingested, the toxic algae could cause nausea and vomiting and rash or hay fever if touched or inhaled.
Beaches along Florida's Treasure Coast were closed last week in response to the expansion of the green sludge.
Though many of the beaches have reopened, officials recommend that people stay away from the hazardous waterways.
Governor Scott blamed the polluted waters on water storage limitations and the federal government.
"Florida's waterways, wildlife and families have been severely impacted by the inaction and negligence of the federal government not making the needed repairs to the Herbert Hoover Dike and Florida can no longer afford to wait," Scott said.
Florida Senator Marco Rubio on Friday called on President Barack Obama to declare a federal state of emergency and to release funds to help businesses affected by the algae.
Mr Rubio and Senator Bill Nelson have also urged the US Army Corps of Engineers to stop the flow of water between the river and Lake Okeechobee.
The Army Corps began reducing the flow on Friday, focusing on the Caloosahatchee Estuary and the St Lucie Estuary.
A cyclist has been killed in a crash on the A82 road to the north of Fort William.
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The prime minister will be curled up on the sofa watching traditional British TV on Christmas Day.
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Swathes of thick, algae blooms have plagued miles of Florida coastline, prompting the governor to declare a state of emergency.