Motorcycle Repair: Honda SL125 engine cuts, camshaft bearings, ohm resistance

Question
Hi, I have a problem where when I turn the indicators on, left or right, the engine cuts. If I rev it the engine tries to die when the lights come on but it will eventually die if left on. What could cause this? I have replaced the battery and that is fully charged. Thanks

Answer
John, it still sounds like a low-voltage problem. First, be sure that the ignition points are clean and adjusted properly. These early engines tend to have poor camshaft bearings, which allow the cam to wobble around during rotation. With the point cam on the end of the camshaft, the point gap starts to fluctuate, causing ignition timing errors. If the cam is loose in the head, then you have a built-in problem that really can't be adjusted away.

Test the battery voltage to the points, while the engine is running. If lower than about 5 volts at idle, then it will have a weak spark, even if everything else is okay. Make sure all electrical connections are clean and tight. If the spark plug cap is still original, test to see if it has either ZERO ohm resistance or 5k ohms at maximum. If there is any tendency for the spark plug to become oil/fuel fouled, correct that problem, too.

Test the voltage output of the charging system, with lights ON and OFF. The lighting switch function incorporates the additional leg of the stator, when activated. A loose wire or bad connection will fail to add the additional voltage to the system when engine is running and lights are ON. That coupled with the additional current draw of the indicators is enough to send the coil voltage below the operating threshold. Test the rectifier for an open/shorted diode leg.

SL125 models in the US didn't have winkers. If you have a Euro/domestic one, then go here:

http://oldmanhonda.com/MC/WiringDiagrams/MCwiring.php#class  use the CL125S diagram for guidance.

If someone has added the winkers, the additional load may be more than the system can supply. Ensure that the bulb wattage ratings are all correct. Looks like they are 18w indicator bulbs and 5.3/25 watt tail light bulbs. 6 Volts, of course. Headlight bulb is a 25/35 watt unit.

Having a out of adjustment brake light switch, where it is either easily triggered while riding or ON all the time will cause the charging system to run out of juice, when coupled with all the other electrical loads.

Bill Silver