Timely Writer has been plagued by bad luck all year, and in Saturday's $400,000 Marlboro Cup at Belmont Park he is encountering more of it.

He is coming into the most important race of his life, the race that could finally legitimize him as a top-class horse, with such inadequate preparation that his prospects of beating seasoned rivals like Perrault, Muttering and Silver Buck seem hopeless.

On Labor Day, Timely Writer was ordered scratched from the Jerome Handicap after a prerace blood test disclosed the presence of a forbidden antibiotic in his system. This misfortune may have been as costly as the illness that knocked him out of the Kentucky Derby, because it wrecked his preparation for this fall's championship races.

Timely Writer had won two minor races before the Jerome, and that stake was designed to get him sharp for Saturday's 1 1/4-mile test. But now he is being asked to come into the Marlboro Cup 24 days since his last start, five months since his last hard race, and go a distance he has never run against a caliber of opposition he has never faced.

The obstacles against him appear insuperable, but trainer Dominic Imprescia still professes optimism. "We hated to miss the Jerome," he said, "and we've been galloping him real strong to make up for it. If anyone can come back and win a race like the Marlboro, it's Timely."

Assuming that Timely Writer is not quite fit enough to win the Marlboro Cup, handicappers know which direction to look for the winner. They should look west.

In the Woodward Stakes Sept. 4, the first leg of Belmont's fall championship series, a lightly regarded California colt named Island Whirl demolished all the East's best horses by five lengths. Island Whirl has not trained well since then and was not entered in the Marlboro, but two superior horses from the West have come here for it -- Perrault and Muttering.

After being sent to this country from France last fall, Perrault established himself as the best horse in California this winter by beating John Henry, 1981's horse of the year. In his last start, Perrault won the Budweiser Million at Arlington Park to boost his earnings for the year to $1.19 million. If he wins Saturday, he will be the favorite to be named horse of the year this season.

The other Western invader, the 3-year-old Muttering, was rested for three months after disappointing in the Kentucky Derby, but came back to win two races in California last month. His trainer, Wayne Lukas, has a knack for getting horses ready for extraordinary performances in major races.