Project Yamaha TX750 Part 12 A question of balance…
Classic Motorcycle Mechanics|September 2021
Only The Beach Boys had good vibrations… so what’s Mark been up to, to sort out the bad ones coming from his TX750?
MARK HAYCOCK

If you have ever ridden aBritish 650 or 750 paralleltwin you will know something about engine vibration. As a passenger it was even worse and I remember being unable to keep my feet on the foot-rests on occasions because the vibration was not just uncomfortable, but painful.

Another effect the vibration had was apparent on a friend’s Tiger 750 and that left a trail of loosened nuts and bolts behind it.

Maybe there was an engineering solution? Norton thought so in the mid-1960s when it introduced the Commando. The engine and rear suspension formed an assembly which was rubber mounted in the frame and this did work on mine. But the assembly needed careful setting up and could allow the engine to bounce up and down if there was a slight misfire.

A better solution would be to eliminate the vibration altogether, and that is what Yamaha explored in the early 1970s. The principle was to connect the crankshaft to eccentrically mounted weights, which would introduce an equal, but opposite vibration which would cancel everything out. The TX500 had the crankpins spaced at 180 degrees and a single weighted shaft was enough to do the job. The TX750, however, had a 360-degree crank and for that two separate weights were used.

On the 750, the weights were mounted on two shafts mounted on needle roller bearings, which were driven by an endless chain from the crank. (Photo 1) shows the balancer and cam-chains on the crank. One balancer shaft rotated in the forward (i.e. crankshaft) direction whilst the other went backwards and this was done by the way that the chain was threaded around sprockets on the shafts. The weights were different sizes also. (Photo 2) shows the larger weight.

The chain would need a way to keep it properly tensioned and a non-adjustable spring-loaded wheel, much like that on the Honda CB750 primary chains, was fitted. Evidently it did not work, as it was deleted after a short time without being replaced. I have seen a reference to a change for the sprockets to have a 1mm offset (i.e. radial, not axial) to take up slack and I wonder if this explains what the four springs we see in Photo 2 are for? The springs are not mentioned in any Yamaha literature; I assumed they were to provide a little resilience in the drive to make it smoother, but perhaps not.

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