Motorcycle racer David Nees of Falls Church may be a bit old at 34, and a bit large at 6-foot-4, 180 pounds, to be living a childhood fantasy. But that's what he will do today in his first pro road race at Lexington. Ohio, riding the most powerful cycle in action.

"I'm still a kid when it comes to road racing," he said. "Growing up in Dayton, I took turns on my bicycle as though I was racing on pavement."

Early this month Nees set a motorcycle track record at Summit Point, W.Va., covering the two-mile, nine-turn course at 83.3 miles per hour on his new Yamaha 750 (45 cubic-inch engine). He sped 150 on the straights.

"I've been racing only five years. Yamaha didn't want to sell me this machine last season," he explained. "It was to go only to expert class riders. And my Cycle Sport Shop is a Yamaha dealer."

Nees wasn't sure he could handle the machine. "The past two years convinced me I could do it," the former school teacher said."The Yamaha costs about $7,500 new but I got a fine used one for something less. It's incredibly reliable and little maintenance is required."

He had tried smaller machines but his size was a handicap in racing against lighter riders. "I had a 250 bike and spent a season crashing and getting used to it. I didn't fit behind the fairing (wind screen) very well either.

"I really want to test myself against top pros."

In today's event he opposes Gary Nixon, former national champion from Cockeysville, Md., and others for a $3,000 purse.