Vintage Dirt Bike Q & A

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PostPosted: Thu Jul 28, 2005 1:52 pm 

Joined: Wed May 18, 2005 10:55 am
Posts: 50
Location: College Park, MD
I originally posted this info on the "cousinweedy" yahoo group list. In case you don't subscribe there I'll repost it here for posterity.

I presume that I am one of the few living graduates of the
American Jawa(NY) CZ service schools, 2 - 2day schools (73 & 76).LOL!!

Anyways, in the dual plug system the coils both unloaded nanoseconds
apart as the sine wave allowed first the rear coil/plug/condensor to
unload then the front plug. That system was discarded circa 1975 in
the second production run of "red Frame CZ's". No loss of HP was
The twin coil&plug system had some intersting plusses and
minuses. If one plug fouled, the other plug was then also out of
service with a 2 coil system. However running one coil with 2 plugs in
the head ( one hooked up at a time), when one plug fouled the other
plug was still servicable, so a quick trackside transfer of the coil
wire put you back into the race. It often then "cleaned" the fouled
plug so that if the "new" plug fouled the old plug would often work
A problem with running dif heat range spark plugs in the same
motor is often hard to understand until you realize that all plugs
basically have the same spark temperature. The dif between spark plug
heat ranges is the dif between the relative "heat sink" properties of
the plugs. "Hotter" plugs are a less effecient heat sink than cold
plugs(compare the electrode position and amount of insulator material
around a B7 plug and a B9 plug and you'll notice the dif.)So, no
matter what plug is hooked to the coil wire, it is the "hottest" plug
in the cylinder that acts as the less efficient "heat sink" . So,
putting a B7hs or B6Hs plug near the exhaust port in a CZ racing
application is an invitation for a "burned" piston. B8H's were the
best all around plug in std. compression CZ MX bikes. Raise the
compression substantially, then you probably should use a B9H range
plug. The B9H range was closer to the original PAL plug supplied by CZ
, but they were too cold for reliable operation. We found that a B77HC
(now obsolete) spark plug worked best for Hare Scambles and Enduro's.
It was slightly hotter than a B8H, but not enough to cause problems
with overheating in those events.
Back in '75 Ivan Boyesen experimented with some CZ's and found no
HP dif on an early dyno between CZ 2-coil/plug and 1-coil/plug
At one of the CZ service schools the Czech Tech was asked how
could one set of points fire two plugs as it was basically impossible
as we understood it to be. His answer when was first translated by
another Czech employee was "The mystery is in the
magnetism"............which BTW is my permenant answer as to why a
bike won't spark. LOL. In further discussions and much blackboard art
he showed us that the flywheel magnets are placed so that two sine
waves are generated by two sets of successive magnets in the flywheel.
These two sine waves both "peak" during the duation of the dwell of
the single set of points. So the first sine wave signals the condensor
in the rear coil to unload the spark and nanoseconds later the second
sine wave peaks, thus signalling the front condensor to unload the
front coil.
Phew , this lesson is almost over. For best life of the CZ points
mount the condensors near the coil rather than in the engine dyno
case. Hook the wire end to the "hot" side of the coil and ground the
coil and it's bracket to the frame. Secondly, we had found a solid
state capacitor from a local electronics store that converted the
Mfu's of the 8v condensor to a rather common modern item ( sorry. "old
Timers Disease has left me memory free of the exact
know. "I'm trying to think, but nothing happens" (Curly-3 stooges). A
little section of heat shrink cover on both ends of the chip-cap wires
and you ground one end and hook the other end to the coil or
points....Bingo, almost an electronic ignition.

Anyways the solid state capacitor made the points almost bullet proof,
as the main contributor to their failure is the degadation of the CZ

Original Dirt

74 AMPI CZ125
73 CZ400
73 CZ250 Basket case.
72 SherpaT 350
75 Montesa King Scorpion
72 Rickman 125 Enduro
70 Honda CL70
72 Honda Z50
00 DRZ400E Baja'd

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