May 20 2007
panzergranate
hotmail
I recently tried out an interesting experiment to prove a theory that
I could convert a CZ 125 into a CZ 150 and it works OK.
You will need a CZ 125 Type 476 or Type 488 engine obviously.
You will also need the following parts.:-
A CZ 175 piston and cylinder block.
A CZ 175 gudgeon pin (wrist pin to anyone in the US!!)
Four CZ 175 stud bolts.
(1) Remove the 125cc piston, cylinder block and stud bolts. These can
be discarded.
(2) Fit the 175 piston and gudeon pin.
(3) Fit the 175 cylinder block with either a new gasket paper or silicon
rubber (more
effective). You will observe that the piston travels up the cylinder
block until 4mm.
before TDC and travels down the cylinder until 4mm. short of being
level with the bottom
of the exhaust and inlet ports. This is not that serious as it will
run with this.
(4) Fit the 125 cylinder head, not a 175 cylinder head.
(5) Tickle the carb and kick start the engine into life.
The bike is now 153cc.
The bike we converted ran up to 59 MPH with a JIKOV 2924 carb.
The engine hits a power restriction at 15 MPH in 1st gear and will not
increase speed.
This is due to the exhaust and inlet ports closing 4 mm. too early,
limiting intake duration.
However the bike does cruise more easily.
So far I've lifted the inlet ports by 4mm. This took 3 hours with various
grinding bits
and a power drill. This bossted acceleration but top speed in 1st gear
was still limited
to 17 MPH. I plan to lift the exhaust port by 4mm. to fix this problem.
I suspect that the early 1970's Aprillia assembled CZ 150 used Asso
Werk piston which may have had cutout ramps in the piston crown as in the
Type 559 JAWA singles of the 1960's.
Only Aprillia made a 150 version of the CZ single under licence, CZ
didn't bother.
This is a simple big bore conversion for any CZ 125 single.
Regards,
Andy.
...................
I tried another cylinder 175 on the CZ 125 and this time only lifted
the
exhaust port by 4 mm. using a reverse cone/frustrum stone in a power
drill working from
the top of the cylinder to take off a 45 degree chamfer to 4 mm. of
cylinder height.
Also I discovered that the Type 488 engine had a CZ 175 head fitted
by the previous owner
after comparing it to a nice Type 476 CZ 125 head I have in the personal
stockpile here.
The bike now pulls 20 MPH exactly in 1st gear before hitting a wall
and needing a change
up into 2nd gear.
The 153cc (58 x 58 mm.) engine has noticefully more bottom end and midrange
stomph than
the 125cc engine did though so far top speed hasn't been tested yet.
It should perform
somewhere in between the CZ 125 and CZ 175 it is expected.
Carburation runs a 98 main jet but with a 50 pilot jet through a foam
replacement
airfilter. Jetting to the 98 mainjet from a CZ 175 is a common mod
when fitting a K & N
or foam airfilter to a CZ 125 anyhow.
The exhaust system is still stock apart from the standard modification
of removing the
large end cap stanction collar to eliminate the flat spot and seal
leak in the rev range.
The bike sounds and runs like a CZ single should.
Anyone else attempting this big bore conversion will need a CZ 175 piston,
rings,
cylinder and studs.
Also a JIKOV CZ 175 main jet/carb and the 50 pilot jet from a CZ 125
carb. The god awful AMAL carb is usually ditched over here and replaced
by a JIKOV anyhow as the bike is 20 MPH slower on the AMAL carb.
Note that post 1985 bikes in the UK have the AMAL fitted usually on
Type 488 and Type 487 CZ "CEZETA" singles.
OK that's all for now on the conversion so far.
By the way, it is the annual JAWA/CZ owners club of Great Britain's
national rally on the
15th, 16th and 17th of June 2007 iat the Cooper's Arms pub campsite,
Pewsey, Wiltshire.
(Near Salisbury). It usually attract hundreds of JAWA, CZ and the occasional
sad MZ
rider, Cossack owner or Voshkod pusher.
For details check out the club's website.
Regards,
Andy.
