Stan Bradbury's Speedway Training Notes
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When removing or installing these magnetos, it is essential that the end of the drive shaft should not be struck with a hammer to remove or install the drive gear or drive chain sprocket. This is because the small bearing at the "Points" end of the drive shaft (rotor assembly) has its position determined by a snap ring set in a rather fragile die-cast housing. A sharp tap on the drive shaft will break the metal of the housing and cause intermittent or total shorting of the electrical circuit within the magneto.
Always use a puller or a piece of 1/4" X 1/2" X 2 1/2" steel stock with 2-8 mm holes drilled 40 mm apart for a Jawa 2V or 2-8 mm holes drilled 2mm apart for a Jawa 4V plus 2-8 mm x 1.25 thread and 1" long hex head screws. Check that there is no damage to the tapered surface of the shaft and, if there is, dress off any high spots with a very fine file then "lap" the drive gear or chain sprocket onto the taper with some fine valve grinding paste. The sprocket or the gear will then grip the shaft even when the nut is only finger tight. When setting the ignition timing, always set the points at 14 thou and then set the timing so that cigarette paper (or 1 thou shim stock) can just be pulled from between the points with the flywheels set right on the correct degree of ignition advance. While doing this, it is essential that any clearance between the mag drive gears or slackness in the magneto drive chain be taken up so that the magneto is trying to "lag" behind the engine (as it would- be during normal operation). Failure to do this will result in an error of between 1 to 5 degrees. The easiest way to take up this clearance is to hang a wrench on the drive shaft nut so that the weight of the wrench will be tending to tighten the drive nut on its shaft. On the 4V Jawa's 894 through 897, the sprocket retaining nut also drives the oil pump. On these models, make sure that the magneto can not come loose and move away from the timing chest. If this should occur, the oil pump will cease operating while the engine is still running, usually with disastrous results.
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