Tuning a Yamaha Bronco engine, part 3

The noise in part 2, after the head work raised the compression, turned out to be the inlet valve touching the valve recess. The recess was machined so the valve clears, and the engine went back together.

Near the top of the rev range, top speed feels much like stock. Compression is up and a CR carburetor is fitted, so torque from a standstill through the midrange is much fatter. I rode it, and the difference is obvious.

Fuel economy has not changed from stock. It is still above 30 km/L.

The head was skimmed 0.5 mm, and the middle 0.5 mm gasket of a three-piece stack was left out, 1 mm in total. That is when the valve started to touch.

A bigger bore is planned later.