Motorcycle Repair: 1977 Honda MR175, maximum gap, ac volts

Question
Bill,

I have a 1977 MR175 with an electrical problem. The magneto is generating power - about 3.5 volts. I have a brand new OEM coil I just put on the bike. When I connect the wiring harness up and kick the bike over I am getting no spark at the plug. I have checked continuity on all of the wires in the harness so I know they are good. The only other option in my mind is the rectifier and I am not sure how to test it to be sure. Could you offer up some advice.

John

Answer
John, the ignition points control the spark timing to the coil and spark plug. Assuming you are using a fresh spark plug for testing and it is grounded against the head/engine, then I suspect that the ignition points are dirty/corroded and/or are not set to about .014" maximum gap. The points need to open at the F mark alignment on the flywheel/engine case mark alignments. The point contact faces must be clean and shiny with no debris caught between them when closed. Also the magnet on the inside of the flywheel needs to be clean and shiny, too. You will need a specific flywheel removal tool to take the flywheel off for inspection of the internals. You can, carefully, clean the point faces through the slotted holes on the flywheel without removal. Use something like a fine grade wet/dry sandpaper followed by contact cleaner to flush out the grit.

I believe that MR175s have two coils in the magneto. One is to power the headlight and the other one is to power the ignition system. The purpose of the rectifier is to convert AC volts from the magneto to DC volts to supply the battery with power. The charging and ignition systems are separate entities. The rectifier does not affect the ignition coil output.

This site (http://oldmanhonda.com/MC/WiringDiagrams/MCwiring.php)doesn't have an MR175 specific wiring diagram, but probably the TL125 version is close to yours, overall, except that the points for the TL are on the camshaft in the cylinder head, not part of the magneto assy on the crankshaft. The one for the Z50A shows the magneto w/points and attendant wiring relationships.

Bill Silver