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John..... ...as you well know, ignition points gap as measured by "feeler-gages" often times leave an oily film contaminate (from sloppy shadetree mechanic techniques) that burns the points and makes pits and peak deposits that a feelergage will miss-read the clearance. A flat ignition point file will clean-off the peak deposit so you can make a more accurate air-gap reading with your feeler. But the fraction of an ohm surface resistance of burned points in 6-volt ignition systems can drastically effect the coil current and reduce the sparkies which leads to missing and other unsatisfactory running complaints. The dwell-meter on the otherhand, can be used on running engines without contaminating the points and it also ignores the pits and peaks of ignition points material transferance due to nature of electrical arcs when the points open. Condensors are supposed to suppress this arcing, but don't always succeed due to technical problems. I always adjust for minimum dwell specifications (widest gap spec). This allows for points rubbing block wear, closing down the gap but still keeps the resultant ignition voltage high enough for good reliable ignition as the normal wear and tear of both points gap and sparkplug gap occure. I agree with Jim, 55-60deg dwell on standard 4-cylinder engines is typical. Its a matter of ignition coil design. Some early dwell meters read in percentage of dwell time. Its a matter of modern terminology. Probably why your Z-120/Z-129/Z-134 engine manuals don't specify ignition dwell (besides being old-fashioned, grin) is 50yrs ago, dwell meters were EXPENSIVE and rare and feeler-gages common. But then torque wrenches were also un-common. 50yrs ago, I was a licensed television station engineer and earned $1.65/hr. I bought my first multimeter for $49.50. (I remember it well, it was expensive) But then gasoline was $0.32/gal..... ..respectfully, Dell
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