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Twin-Engined Triumph Bonneville

1974 Triumph Bonneville

Steve Pieratt’s amazing twin-engined Triumph Bonneville. It’s long, but it still looks like a Bonnie.

The annual Barber Vintage Festival is always full of surprises. Interesting, odd and downright weird classic and vintage bikes seem to come out of the woodwork for this annual celebration of all things motorcycle, including machines like Steve Pieratt’s incredible twin-engined Triumph Bonneville, which Steve entered in the Motorcycle Classics Vintage Bike Show at the 10th Annual Barber Vintage Festival in 2014.

Based on a 1974 Triumph Bonneville 750 T140, Pieratt’s bike has been extensively modified to accommodate a pair of 750cc Triumph twin engines seamlessly mated together, resulting in a 4-cylinder, 1,500cc Triumph Bonneville. The two engines share a common primary drive, which couples the front engine (which has had its transmission surgically removed) to the rear engine. The rear engine retains the stock Triumph T150 5-speed box. The primary cover was created by grafting two stock covers together, welding and shaping them to the desired profile, followed by final polishing. The finished cover is a work of art and looks like it came out of the Triumph factory. A polished stainless steel plate couples the two engine case together on the right timing cover side.

1974 Triumph Bonneville

A close look at the engine shows a pair of stock primary covers welded and formed together; they almost look factory.

The cleverness of the adaption is more than a little intriguing. Just bolting two stock Bonneville engines together would require an especially long power unit. Room would be required for separate carburetors for each engine, along with the necessary space for the exhaust headers on the rear engine, which would sweep toward the front engine. In practice, they would have to be kept as far back from the front engine’s carburetors as possible to keep the heat of exhaust from boiling the fuel in the front carbs, and to give each engine air flow for cooling. To address that, the cylinder head on the rear engine has been reversed. As a result, the exhaust headers on the rear engine exit to the rear and the intake ports of both engines face each other. That allows fitment of a pair of custom intake manifolds, with the right manifold and carburetor feeding the right cylinders of both engine and the left manifold and carburetor the left cylinders. Carburetor setup and tuning is no more difficult than on a standard twin-carb twin.

The kickstarter lever has been lengthened to clear the exhaust, and to give the rider plenty of leverage when kicking the two engines over. And kick over they do, as we witnessed when Steve fired it up, to the delight of the crowd. Click here to see the video of Steve firing up the Triumph.

1974 Triumph Bonneville

Right side view shows the long kickstarter lever. It makes starting the pair of engines amazingly effortless.

The twin-engined wonder not only starts and runs, it’s a rider, too. I had just walked up during Steve’s start-up demonstration, and he offered me a quick ride on the twin-engined Triumph, giving me the entire experience, from starting it to riding it. The engine was already primed from having just run, but Steve claims it always starts easily, as it did for me. Expecting huge resistance from the two twins, I heaved into the kickstarter, only to have it swing down with relative ease, the two engines burbling into life almost effortlessly. Wow. And it idled. Just fine. I expected a long reach to the bars, but they pull back far enough to fall fairly naturally to hand. Instead of the comical experience I expected, it felt fairly normal. The transmission shift normally and properly — on the right — and feeding in a little throttle and letting out the clutch, the twin-engined Triumph pulled effortlessly way, the torque of the combined engines allowing rapid clutch engagement at slow engine speed and a smooth, snatch-free transition. I only ran it around the vendor expo area perimeter road, so I never got it out of second gear or up to any kind of speed. Even so, Steve’s twin-engine Triumph surprised and impressed me. Beautifully made, it’s a real bike you can actually ride. — Richard Backus