When the Japanese manufacturers entered Grand Prix roadracing in 1959–’60, the ultimate in carburetion was Amal’s “GP” series of rigid-mounted smoothbore carburetors fed by separately mounted remote float bowls. Mikuni in Japan supplied Yamaha with its racing M-type carburetor, which, like Amal’s aluminum GP, was rigid-mounted to engines and fed by a separate float bowl.

Engine vibration was always an issue, at first forcing users to hang their remote float bowls from flexible rubber elements and then, in the case of Gilera and MV Agusta, to also mount their carburetors with some decree of vibration isolation on short pieces of fuel-compatible rubber hose held in place by hose clips. Moto Guzzi, in its fabled 500cc V-8, tried at first to fuel all its eight interdigitated carburetors from two float bowls—one on the right, one on the left. This worked so badly (Pop! Bang! Kakkachooie!) that Guzzi itself (already making the carburetors) ultimately produced sets of eight tiny separate float bowls, one per carburetor.

Belgian Grand Prix
Fumio Ito (26) won the 1963 Belgian Grand Prix 250cc race at Spa-Francorchamps. Factory Yamaha teammate Yoshikazu Sunako (54) was second.Courtesy of Yamaha

The used 1966 Yamaha TD1-B production roadracer that I paid $500 for in 1966 carried two rigid-mounted and Mikuni-made zinc-alloy carbs that proclaimed from molded-in lettering "AMAL 276." They were served by a pair of cylindrical float bowls that gave new meaning to the expression "remote floats," for they were several inches behind the carburetors. This meant that during acceleration fuel sloshed to the rear, away from the carbs, leaning out the fuel-air mixture, and during braking sloshed forward, flooding the carbs. The two floats were held in anti-vibration mounts of bonded rubber but bolted to the top rear of the engine crankcases. Since 180-degree-firing parallel twins have a vigorous rocking motion, the rather stiff rubber mounts were less-than-completely effective. Well-informed riders of that time corrected this by mounting a single Amal float bowl between the carburetors, thereby eliminating the rich/lean effect.

In 1967, Yamaha upgraded its float-bowl mounting by hanging the bonded rubber mount from a bracket, attached to the chassis under the fuel tank. This was an improvement, one of many that made the TD1-C into quite a good machine.

Mikuni now developed its famous VM series of concentric integral-float carburetors, which mounted flexibly via a bonded-rubber intake stub bolted to the engine. Early versions began to appear on factory racebikes in 1964.

In July 1963, Yamaha’s factory team—in Europe with the new RD56 air-cooled twin and riders Yoshikazu Sunako and Fumio Ito—were unable to get their engines to accept full throttle on the long straights of Belgium’s Spa-Francorchamps race course. These machines had rotary-valve intake so the two carburetors projected to right and left. Each carb had the usual single separately mounted float chamber. In desperation, the team cobbled together a mounting that gave each carb two float bowls—one ahead and one behind it (so they had the main jet well-surrounded). This quick fix was evidently able to overcome the effects of the engine’s rocking motion well enough to sustain full power, with which the two Japanese riders easily cruised away from the following Moto Morini and four-cylinder Honda four-strokes. Race winner Ito was 22 seconds clear of the first four-stroke, Tarquinio Provini’s highly developed Morini single.

That was evidently enough for the engineers: no more 1930s carburetors ever again! Mikuni developed its famous VM series of concentric integral-float carburetors, which mounted flexibly via a bonded-rubber intake stub bolted to the engine. Early versions began to appear on factory racebikes in 1964. In 1969, Yamaha’s truly excellent 250cc TD2 production racer replaced the TD1-C and sported a pair of integral-float VM30s. Millions of VMs thereafter served motorcycling for decades.

Honda had already traveled a similar route. The 1960 250cc four-cylinder RC161 factory roadracer still had rigid-mounted carburetors, each with its own narrow separate float chamber beside it, but the problems with these quickly led to development of Keihin integral-float carburetors on vibration-isolating flexible mounts.

Interesting to note that Oscar Hedstrom first examined an early de Dion gasoline engine in 1899. He was evidently not satisfied with its crude evaporator carburetor for in 1900 he designed his own spray carburetor with an integral concentric float bowl. Hedstrom then designed the first Indian motorcycles.