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Motorcycle Repair/'77 Honda XL100


I rescued a 77 XL100 that was otherwise headed to the junk pile. The wiring was trashed. I constructed a rectifier with some parts from Radio Shack, but I don't understand the ignition coil that has only one primary wire. And that wire goes to the points. Where does a hot wire come into the system? On the same wire? If so, there will be a direct short from the battery (and charging system) every time the points are closed. Every wiring digram I can find shows 2 primary wires.
I wired (the only primary wire) to one of the stator wires upstream of my rectifier, and the coil would throw a good spark. The engine starts right up, but it won't run above a fast idle---because the spark won't speed up. Can you tell me exactly what needs to be connected to that single primary wire in the ignition coil, or am I missing something else?

David, the later XL100-125s had more of an energy transfer ignition than the earlier battery powered version.

Looking here:
there should be a double connection that the coil plugs into for the two connections.

When I looked up the specific coil for the 77 XL100 I saw that it also fits the TL125 series which uses a similar ignition system...

The ignition is powered by the primary coil underneath the flywheel, instead of a battery. When the engine spins over the power is generated up to the secondary coil which is then managed by the points/condenser.

Do not attach anything else to the ignition circuit apart from a kill switch. The charging system wires coming from the stator are to charge the battery only. The Black/White wire is for the ignition coil function. If you tie in the charging coil wires, things will go south in a hurry.

Once you have this sorted out, check the point gap (O.014"), the plug cap for excessive resistance over 5k ohms, set the ignition timing so the points just open at the F mark alignment on the rotor. Finally inspect the spark advancer unit to ensure that the point cam is free to advance/retard properly.

Make sure the carburetor is cleaned out properly to ensure fuel flow off idle speeds...

Bill Silver

Motorcycle Repair

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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