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-display the power and controllability of their E.C. I;:rt-prepared 250s at Indian Dunes' motocross course near Valencia, Calif.
by John !luetter with E.C. Birt
and Jim Hoover
There is trick and then there is
super·trick in the world of preparing
racers. Just when you thought you'd
read all there was (or you were ready
for) about supertuning and preparing
your CZ, here comes another technical
hot set-up at you.)t only goes to show
there are no absolutes.
StocK Head Configuration
Something very close to the ultimate 250
set-up. A Mikuni with a reed valve on one of
E .C.'s engines.
It is an interesting but little-known
fact that £.C. Birt, noted for his 125
work. also does the number on 250cc's
and up. orne of his more successful
250 preparations 'have been CZs. For
instance, the one Tony Rogers has
ridden to a 0-37 motocross number one
plate and the one that John DeSoto
rode in winnjng Elsinore in 1971.
He just super-built a 250 CZ for Greg
E.
c. Sirt APproach
Balanced flywheels are prepared by Jim
Wilson·s dad. A Femsa transitorized ignition is
another option.
Lee who then won the AM£ CZ series
and ... another machine of the same
marque. There are "stage" approaches
to the 250s.
Birt piston ports the 250 for riders
who want it, retaining the same ports
but enlarging the dimensions to. thc
rider's ability. Who will ride each bike
and how it will be ridden largely
determine the extent of the hOp-up and
preparation that E.C. puts into it. Also,
how much green you can afford to part
with.
A 34mm Mikllni is used on the 250s
and Lee's 250 also had a 011 reed valve;
more about that set-up in a few lines. A
custom E.C. pipe was mounted on the
young hotshoes' 2505. Each pipe is a
little different because each of the
scooters is a little differen tly tuned.
The pipes incorporate silencers, of
course.
After the carbureting and porting,
E.C. dares where others have not
ventured (possibly with good reason)
and mod ifies the shape of the
combustion area in the CZ head. (See
fig. 1) Tile intent and apparent eHect
of the headwork is to move the
combustion area to the area of
maximum compression; thereby
focusing the flame front under the
center of the piston.
One result of the head mod,
according to E.C., is more power
throughout the RPM range, not Just at
top end or low end. Actually, most of
the Birr set-ups are to give a bike more
power throughout its entire range of
revs. Reed valve, Mikuni and special
pipe all work to have that same effect.
If it's done right, they should
complement each other. Birt seems to
have a knack for making the whole
package work.
The great CZ ignition timjng fallacy
has been exposed! Or, at least, E.C.
takes a djfferent approach to the
subject. All the 250s have the timing set
at 3mm BToC. Maybe back off to 2.9
01; a 125. Advancing ignition timing is
part of the game to get more torque at
the low end out of a low horsepower
engine. The Birt approach is to get a lot
of horsepower out of a racing engine in
the first place and then not screw
around with stuff like timing. You
thereby avoid timing discussions.
Riding the 250s, in sequence
matched to stage of preparation, is quite
a revelation. You can start to get a feel,
even as a novice rider, of the kind of
machinery the number one plate
demands. Tony Roger's bike is set up
especially for him, tailored to his
requirements. Externally, it differs from
Lee's bike in lacking a reed and
featuring differen t shocks. The bike is
qukk and very strong but you do have
to gct going rather fast to get it on the
pipe. Needless to say, it is geared
appropriately for the speeds the engine
will propel it. You still have to wick it
out of the corners and there is a definite
rush of power.
Greg Lee's 250 had the same set-up
plus the 0)-1 reed valve. The fe.el out of
the corners is more like a 400. That reed
attached to the Japanese air-fuel mixer
really smooths out the power, dragging
some of the horses down in to the lower
RPM range. You don't have to keep the
250 buzzing quite so hard to get good
results. This is, naturally, most obvious
out of the corners. The trick engine is
smooth and very strong. It also sounds
something like a turbine as the intake
mj~ture whines through the reed. It is
