Re: JD 520 missing when it gets warmed up
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Posted by MattT (OK) on August 04, 1998 at 10:17:37:
In Reply to: JD 520 missing when it gets warmed up posted by Tom on August 03, 1998 at 06:13:27:
: The last 2 times I have been bushhogging the tractor starts to miss after it gets good and warmed up (~30 minutes on a day thats 80 degrees F). It will be running fine then it will miss a few beats and make a loud pop when it does fire.At this time I limp it back to the garage, It will miss a few times even when the bushhog is not running. The fuel bulb is full and clean when it happens. The coil is the only old part in the ignition system. I pulled the plugs the look good. Is the coil a good place to start? Is it necessary to get a JD coil or would any coil made for a points and condensor 12v system work?
: Thanks for the help
: Tom Wyble in western NY Tom, something else to check would be the plug wires. I have had almost exactly the same symptoms and it turned out to be a plug wire. An electrical engineer friend of mine explained it this way. The plug wire has a given resistance when new. As they age the resistance goes up. The resistance also goes up as they heat up causing the miss when warm but not when cool. Through age and increased heat from the increased resistance, cracks develop in the wire insulation causing the "pop" (actually backfiring through the carb on mine) on a different cylinder when the spark would jump to ground. We found the crack by operating the hot motor in the dark and seeing the arc. After pulling the old wire I could not even see the crack but the spark was very evident in the dark. Hope this helps and doesn't add to your frustration. MattT(OK)
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