Yamaha
TZ250
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At the start of 1972, Yamaha Factory mechanic
Ferry Brouwer fitted a pre-production watercooled
top end set-up to "Brit" Chas Mortimer's
TD3. The paddock was buzzing with excitement
at this revelation and by the time the GP season
had kicked off, several other leading riders
had appeared with the same equipment.
The TZ250A came out in June 1973 and, like
the 350, was virtually unchanged porting wise
from the previous aircooled racer apart from
a slight widening of the main transfers, also,
the exhausts were unchanged.
Unlike the TZ350, Yamaha chose to have built-in
automatically variable ignition advance in it's
Hitachi TIA02-16 CDI unit. It was programmed
to advance linearly from the initial mark alignment
static to a maximum advance of typically 14
degrees at 3000rpm. It then returned slowly
and linearly as the revs increased, reaching
the original alignment set point again at 10,000
rpm. Beyond 10,000 rpm it continued to retard
at the same rate at which it approached 10,000
rpm. (i.e. 2 degrees per 1000 rpm)
Yamaha also chose to "beef up" the
main and small-end crank bearings as well as
a few other minor mods . Output was up slightly
from the TD3 to 51bhp @ 10,500rpm.
"The following year's model, the "B"
was virtually identical to the "A".
( '74 and '75 must have been very frustrating
times for TZ customers ! )
1976 saw the appearance of the much awaited
TZ250 C. The biggest news here was the new "Monoshock"
chassis, with the rear shock featuring adjustable
pre-load and rebound damping, as well as disc
brakes front and rear. Motor-wise, the factory
chose to pursue ease of riding rather than outright
horsepower. Revised porting comprising wider
intake and transfer ports with altered angles
of entry, raised compression and a new exhaust
achieved this for them to a certain degree.
The exhaust was mounted on new design brackets,
and the clutch basket "boss" to primary
driven gear attachment was improved by changing
from previous model's "dog" teeth
to a female spline, this of course required
the primary driven gear to be altered to utilise
a male spline. The bike sold remarkably well
at just £ 1,500 including a well stocked
spares kit.
The "D" model of 1977 was just about
identical to the previous model except for the
inclusion of revised heads with angled spark
plugs and new expansion chambers with silencers
as standard issue. Minor changes included the
side gearbox drain bolt gasket, 2 crank web
halves, base gasket and the centre crank bearing.
Carburettor mods including new floats, float
chamber gaskets and choke systems also appeared.
Peak power hopped up by 1 to 53bhp@ 10,500rpm.
The factory's 1978 250cc offering, the "E"
was a little disappointing in that it was virtually
unchanged from the previous year's offering,
apart from a new chassis which had the engine
sitting approx. 20mm further forward, the bike's
swingarm remained the same. The front of the
engine was retained by a pair of alloy mounting
plates which required an additional 20 or so
bolts, nuts and washers to secure to the engine
and frame. The rear lower engine mounting bolt
was also lengthened by 5mm.
The TZ250 F heralded welcome news for Yamaha
fans when it was released in 1979. A 11kg weight
reduction was achieved through further changes
to the frame, (despite the fact they reverted
back to welded steel engine mounting brackets
from the alloy ones on the "E") including
a steeper rake and an alloy swingarm, as well
as this the bike featured a new exhaust and
mounting system, an additional o-ring (from
the previous single o-ring) in the header pipe,
new conrod, wider intake port and minor carburettor
mods. The primary drive gear picked up 2 teeth
to match the 350's 25, the cylinder drain tube
was altered. A dipstick was also introduced
and the brake calipers became cast alloy units
like the 750's . Power remained the same as
it had since the "D" at 53bhp @ 10,500rpm.
A new fairing adorned the machine which was
pointed below the header pipes and eliminated
the old aluminium belly pan, a new fuel tank
and seat unit appeared, and the clutch cover
was reduced in size to a pressed aluminium job
(the previous models utilised a cast aluminium
unit completely covering the clutch) surrounding
just the rear of the clutch unit.
 1980's
"G" model was a further departure
from the "norm" of the past 3 or 4
years. New, larger diameter forks were fitted
in an effort to combat the front end "patter"
problem that was frustrating so many riders.
Motor-wise Yamaha went all-out for peak horsepower
at the expense of engine lifespan by drastically
increasing the inlet timing period by removing
almost 8mm from the rear of the pistons (and
slightly increasing bore size) . This mod caused
real problems with piston wear, despite the
factory's attempt to counteract it with the
inclusion of a short supporting "tongue"
protruding down from the top of the inlet port
in a vain attempt to reduce piston rocking.
TZ250 G's were, as a result, very expensive
to maintain, requiring re-builds after every
race, though no-one complained about the extra
4bhp it provided over the D/E/F models ! Yamaha
also increased the width of the secondary transfers
slightly this year as well as fitting powerjet
carburettors.
The 1981 "H" represented the factory's
first departure from the popular 54mm bore x
54mm stroke (essentially) configuration of the
previous 9 years along with a switch to an individual
cylinder casting configuration. In an effort
to further boost horsepower, the bore was increased
to 56mm and stroke reduced to 50.7mm, achieving
the full 249cc in the process and mechanical
exhaust power valves were fitted, as was a transmission
oil pump to deliver the reduced oil capacity
of the gearbox to crucial points. The "H"
had a 38mm shorter motor than the "G"
and 36mm ZC series Mikuni carbs replaced the
usual 34mm SC's, which had been in use on the
250's since the TD3 way back in 1972. Intake
port was again widened, as were the secondary
transfers for the second year running.
The motor spun in reverse direction as the
factory tried to further reduce the stress on
the rear of the pistons. The crank was no longer
a single unit, it was now a pair of cranks with
a gear on the end to deliver the power to the
gearbox. Despite the frame shedding 3kg, the
overall weight was reduced by just 1kg. The
steering rake was altered and the swingarm also
lengthened to compensate for the shorter frame
length of this model. The engine was now mounted
slightly further towards the front of the bike.
The new rear shock became fully adjustable for
both compression and rebound damping as well
as pre-load and the shock picked up a rear rubber
mounting. The fork diameter went up by 1mm and
brake wise the factory reverted back to the
old cast iron calipers which didn't bend under
pressure and also didn't fade when hot. The
new ignition dropped an igniter and pulser coil,
there was now just one of each and the inner
clutch hub now became a single piece unit to
eliminate one reliability problem from previous
years. The un-necessary neutral switch mounting
disappeared
 The
"J" of 1982 was virtually identical
to the previous model apart from a few minor
"glitches" being sorted out from the
"H".
Information kindly provided by www.tz350.com
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