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CZ Twin Coil Ignitions Explained http://vintagedirtbikeforums.alp-sys.com/viewtopic.php?f=39&t=731 |
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Author: | original dirt [ Thu Jul 28, 2005 1:52 pm ] |
Post subject: | CZ Twin Coil Ignitions Explained |
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 detected. 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 fine. 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 systems. 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 number.....you 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 capacitor. Original Dirt |
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