Race Track Refugee: Harley-Davidson XRTT-750

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by Dain Gingerelli
Harley-Davidson XRTT-750 converted to a street bike.

What do you do with an aging race bike that’s obsolete, yet hasn’t quite reached collectable status? Well, you could convert it to a street bike. That’s what Ron Martin did back in 1977 with a Harley-Davidson XRTT-750 rolling chassis that had been mothballed from road race duty shortly after Yamaha unveiled its TZ700 in 1974.

The moment that the banshee wail of that legendary 2-stroke inline 4-cylinder engine echoed off Daytona Speedway’s banking, the writing was on the wall for the antiquated XRTT-750 V-twin, or any other 4-stroke race bike, for that matter.

Martin, of Seminole, Florida, had acquired his relic racer when it was still intact, less engine. The road race bike originally cradled Harley-Davidson’s XR-750 engine — in either the early iron-cylinder incarnation or with the alloy cylinders and heads that debuted in 1972 — before the weekly tragedy that’s better known as professional racing relegated the venerable V-twin engine to the sidelines.

Plain and simple, speed caught up with the XR-750 road racer, and the slogan “go fast or go home” applied then as it does now; anybody competing in road racing with a 4-stroke-powered bike back in the mid-1970s was wise to go home rather than face the futility of competing against the powerful 2-stroke racers that shredded race records almost as quickly as they shredded their race tires. Ironically, the last 4-stroke to win an AMA National road race during the halcyon 1970s was the XRTT-750 ridden by Cal Rayborn when he smoked the competition at the 1972 Laguna Seca National.

Slow drift

Sadly, too, the path to obsolescence for Harley-Davidson’s XRTT-750 was a rather slow, arduous and somewhat painful one. The saga began in 1970 when the AMA put its new overhead valve 750cc rule into full effect for road racers, as well as for the flat track bikes that the rule applied to beginning in 1969. Prior to 1969, the AMA implemented an equivalency formula stacking bikes with sidevalve 750cc engines against competitors using 500cc overhead valve engines. In the years prior to 1970, Harley-Davidson relied on its trusty sidevalve KR-750 to wage war on both paved and dirt race courses.

Indeed, Cal Rayborn and his KRTT-750 dominated on Daytona’s banking two years in a row, winning the 200-mile race in 1968 and 1969. In those two years nobody else really came close, not even a young Canadian named Yvon Duhamel aboard his Fred Deeley-sponsored Yamaha 350, among the first 2-stroke racers that began knocking on the 4-stroke clubhouse door.

Then, for 1970, a slew of powerful multi-cylinder 4-stroke road racers began to fill the grids at AMA National road races. Chief among the new breed of racers were the factory teams of BSA and Triumph, populated by a small army of 3-cylinder 750s ridden by some very fast racers, among them former Grand Prix great Mike Hailwood. Honda showed up, too, with a bevy of modified 750 fours, and those ringy-dingy Yamaha 350s and Suzuki and Kawasaki 500s were getting persistently faster, too.

Harley’s response was to adapt its iron-cylinder Sportster engine to Class C racing rules. Essentially, Harley’s race team began the transition by basing the new 750cc engine on the XLR, a competition-bred 900cc variation of the street engine that had been in use for several years already.

Chief engineer Peter Zylstra was tasked with the XR-750 project, and the solution was to de-stroke the XLR to 750cc specs. Leaving the bore untouched allowed Harley to retain the XLR’s large intake and exhaust valves — more fuel and air in means more power out.

Naturally, there was more to the transformation than simply downsizing the engine, and during the journey to create the first XR-750, the protagonists — that is, the engineers — found ways to improve (read: add more horsepower) on the XLR’s design. According to lore, when the new race engine’s cylinder heads were cast, slightly manipulating the casting boxes during the pouring process allowed engineers to “accidentally” enlarge the intake and exhaust ports. Again, more air in for more power out, which was just fine by the racers.

The iron-based XR-750 engine proved to be a temperamental, yet worthy, stop-gap solution that allowed Harley’s race team engineers time to regroup and develop the all new alloy-based engine that would be even more powerful.

That engine debuted in 1972, and although it retained the XR-750 label and it shared the same basic design parameters — a 45-degree Vee trough between its two cylinders, fork-and-blade connecting rods, and unit case construction housing the bottom end and transmission together — the alloy engine boasted design components not possible with the old iron engine. It was, for all intents and purposes, an all-new engine, with its own bore and stroke dimensions as well. In addition, the alloy XR-750’s cylinder heads were fed individually by two Mikuni carburetors mounted on the right side and tucked out of the turbulent airstream, while the exhaust ports were conveniently positioned into the wind for additional cooling effect. This engine was built for speed, with few of the compromises that had been dictated by using the street-based Sportster engine.

As history has shown us, the new alloy engine proved a success, and lives on to this day winning flat track races across the country. It was even responsible for several road race wins, including Rayborn’s heralded 1972 win at Laguna Seca. But even so, the 4-stroke V-twin was living on borrowed time as a successful road race engine. The Japanese 2-strokes were getting faster and faster, and their engineers were even beginning to understand the credence, “Build it fast and build it to last.” By 1974 the speed and reliability of those oil-burners had exceeded anything possible with a 4-stroke. It was more than a coincidence that the words “camshafts and valves” were seldom heard in the pits at road race tracks during those years.

Ron Martin’s XRTT

Still, the XR-750 had never lost its edge on America’s dirt-packed oval tracks, and more than likely the original race engine in Martin’s refugee/racer street bike had been conscripted for use there. Replacing the bike’s race engine for street use actually proved to be a rather easy proposition — Martin simply shoehorned a stock Sportster engine in place, electric starter and all, before he continued the bike’s transformation.

After plumbing in a set of exhaust pipes with chromed megaphone tips, he wired the electrics to include front and rear running lights. To conform to Florida’s vehicle codes, he cut a small hole in the Wixom Brothers fairing’s nosepiece to make way for a compact Windjammer headlight, and he tucked a small taillight with license plate holder within the bike’s tail section. As a further token gesture for legality’s sake, a Dick’s Cycle West mirror was discreetly nestled within the confines of the fairing’s left side. The bike’s RPM Engineering tachometer was spared, however, and there’s no street-worthy speedometer to be found on this race track refugee. The classic 18-inch rims were shod with Dunlop KR-series race tires, but for the record, most Harley factory road racers rolled on Goodyear race tires.

When the metamorphosis to the street was complete Martin’s friend Roger Nelson applied the orange, white and black paint. Fittingly, the paint scheme mimicked that of the Harley factory race team bikes, and it was refreshing to see the Number One plate on a Harley-Davidson road racer in 1977, a time when most pro road racers relied on the TZ750 for their success.

That’s when I encountered Martin and his repli-racer street bike. I was at Daytona International Speedway preparing to photograph the 1977 Daytona 200 when I spotted the ex-XRTT racer in the vast infield. I introduced myself to Martin, and he obliged to let me photograph the bike in between practice sessions one day.

I was editing Hot Bike Magazine during that time, and the bike enjoyed a two-page black-and-white spread in the October 1977 issue. My report included mention of the XR/XL garnering several awards at the Bayfront Center Bike Show held in St. Petersburg, Florida, earlier that year. It was during a recent scrounging session through my photo files that I stumbled upon these old black-and-white photographs (anybody remember Kodak Tri-X film?). Unfortunately, my notes mysteriously disappeared, so some information about Martin and his bike has been lost over time.

Moreover, Martin’s bike went up for sale shortly after I took these photographs, so I never knew its fate after the magazine article appeared. Hopefully the bike is in someone’s care today. The wild child residing within me feels that the bike belongs on the street, while the rowdy boy racer who stirs within my soul prays that the relic XRTT-750 found its way back onto the track where all race bikes belong. Anybody out there have a lead on this? MC

A former editor with Hot Bike Magazine, Cycle Guide and four other publications, author Dain Gingerelli was a competitive motorcycle racer from the 1960s through the 1980s and co-held four FIM world endurance speed records in 1986. Ironically, he earned his “Daingerous Dain” nickname racing cars. “Yeah,” quips Daingerous, “that was a dark time in my life.”

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