In 1973 Yamaha introduced the TZ series.
From 1973 to 80 the TZ250 & 350 used a piston
port parallel twin monoblock
design based on DS7/R5 cases. The TZ250G being the fastest of
the monoblock 250s
with an intake so big they had to extend the intake port roof down 4mm in
middle for piston/ring support. Piston life was short but it sure made power.
Fastest of the
TZ350s were the F&G with their 6 transfer port cylinders.
TZ350 6 transfer
In 1981Yamaha totaly redesigned the TZ250 engine. Having
reached the limit of intake port size
in the G, the H models crankshaft rotation
was run backwards to put piston thrust on the cylinder
wall opposite the intake
allowing further increases in intake port size. Also new was Yamaha's
power valve system which in it's infancy was having teething problems. Two
Hans Hummel provided a solution by producing his own non power
Hans Hummel TZ250
In 1985 Yamaha again Redesigned the TZ250 engine. 1985-1987.
Switching for the first time to
case reed induction allowing for much needed
transfer area (2 new boost ports were the intake
once was) creating needed
bottom end & thereby also a wider powerband.
In 1988 through 90 Yamaha reversed the cylinders & tilted them forward to
60deg putting the
carbs in front & the pipes coming straight out the back
under the seat.
In 1991 Yamaha went to the V-twin design allowing room for
good transfer design while letting
the crankshaft & engine to become