1980 RM125 No Spark Potential Low Comp

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12/18/2018 9:35 AM

Hi, I have a 1980 RM125 with no spark and its getting about 135-140 psi of compression. For starters, is that good or should it be rebuilt and secondly any tips for finding whats causing the spark to not work? A checklist perhaps? Thanks.

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12/18/2018 10:32 AM

Your compression is low. Replating the cylinder and a new piston should take care of that. Youre looking for 160+ psi.

As far as the ignition. Start at the plug, then coil, ground, cdi, stator. Sometimes the stator can short out inside or the flywheel can lose its magnetism.

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12/18/2018 10:47 AM

1980 rm125s have a steel lined cylinder you can just have it bored, no need for plating... Personally, I would disconnect the kill switch first and see if it cures your lack of spark issue... then proceed with the list KB has suggested for you...

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12/18/2018 12:22 PM

Your compression will be ok for now to get it running, but will need to be refreshed later when the compression drops below 120psi.
RE: Spark....
1. The coil should be OHM tested
2. The plug wire and plug cap become corroded over time. Unscrew the cap, cut 1/4" off the wire and then screw the cap back on.
3. The coil mounts become rusted and thus you loose a ground. Remove the coil, sand the mounts on it and the frame and try to get spark again.
4. Those bike were known to loose spark when dust and dirt gets inside the stator cover and cause rust on the flywheel. Remove cover and pull flywheel and clean. You will need a puller to do that, so do the other stuff first.
5. If all above fails, then do an OHMs test on the stator.

Paw Paw

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12/21/2018 1:36 PM

Paw Paw above steps are right. I can add from owning 79-80 RM125s,,,135-140 is a good compression number on stock head. Gauges can vary a tad. 140 is stock psi though. 160 comes with a high compression .020" head gasket and DG head. Racing my 79 RM125 a few seasons taught me the High speed coil on stators goes first...it'll start popping after few minutes racing and screaming when it fails. Let it cool off and work for a few more minutes...The trigger coil didn't fail. That gives the initial spark at low rpms. Others that had this model rm told me they experianced same. So I keep a rewound stator from Custom Rewind just in case. Also ,,I remember the routing of stator wires around frame can rub through and ground out too,,something to look for as well.

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