If your Motobecane stock speedometer is not "melted" with dark bubbles in the corners from blown-out bulbs, please disconnect the bulbs with an electronics flathead screwdriver. Especially if you hardwired your Moby to mitigate the shoddy stock electrics. You'll save yourself future grief.
My problem was that I had one melted but functional speedo, another 60 mph frozen speedo, and yet another speedo with intact faceplate and drive, but a broken needle and rusty chrome.
The melted face is ugly to look at and it caused me displeasure. So I took the needle and chrome from the frozen one, and the housing, faceplate, and drive from the rusty one, and made a nice working one.
In the process I learned how to service these beautiful but obnoxious devices. Pardon the pics of broken speedo parts. I won't open the refurbished one just to photograph it.
Disassembly and cleaning
First I disassembled and cleaned the housing. There was a 17 mm nut holding the actual speedo in the housing. Under that, a fat washer that shielded a brass nut. Finally, a V-shaped strip of metal that expands when tightened.
I removed the nut, washer, and locking piece and polished them with Blue Magic and #0000 super fine steel wool. Then I broke the gasket between the chrome and housing and polished the speedo case.
This removed much gunk and let me pull out the bulbs in the top corners. I washed the housing with soapy water.
Cracking open the speedo
I opened the speedo by removing the brass nut with a crescent wrench. The chrome, glass, and gasket assembly was tricky to remove and replacements are only sometimes available if you break the glass.
The trick was to go very gently around the chrome's bent tabs with a mini crowbar. Very gently, because it's easy to break the glass at this stage. If you break the glass, it's possible to cut new glass from a 2 mm thick sheet.
When the glass and the brass nut were removed, I could slide the speedo out of the case and begin working. The magnet was encased in an aluminum shield.
DO NOT MANIPULATE THE MAGNET CASE, BECAUSE AN EXTREMELY DELICATE SPRING LIES DIRECTLY UNDERNEATH. It will deform under even slight pressure.
Separating the speedo and odo
To service the internals, I separated the two main functional units: the drive and the faceplate. There were two 5 mm bolts on either side of the cable input. I removed them with needlenose pliers, not having a socket thin enough to fit. I pulled the magnet straight out and kept the faceplate with magnet case, needle, and the EXTREMELY DELICATE SPRING in a safe place.
Huret used a terrible grease in their units that has since hardened into glue. That grease is probably why your Moby's speedo or hub drive is broken in the first place. Fortunately, some units were never greased enough to cause a problem.
If there's any hard red grease in the gears of the drive, dissolve it with a strong solvent. I had partial success with naphtha (lighter fluid) and would recommend acetone (nail polish remover). Then I gently lubricated the moving parts with light oil and removed the excess with a Kimtech wipe.
At this time I tested the drive's function. I inserted a speedo cable and manually twisted it counterclockwise. The odometer turned over smooth and slow.
Replacing the needle
I gently teased apart the area between the raised Motobecane logo and the faceplate with the actual speed marks. You may want to heat the welded plastic with a hot nail because there's a good chance that cold plastic will crack.
Now the needle was accessible. To remove it, I gently pinched below the needle base with needlenose pliers and pulled straight up. I didn't attempt to twist it off because that would ruin the EXTREMELY DELICATE SPRING on the other side.
Then I manually pushed a new needle onto the spindle while the spring was at its natural tension. I took care to position the needle at zero. That's the baseline speed, even if you're a high roller.
Putting it all together
Finally I reassembled the speedo/odo internals and quickly tested it again with a cable. Turning counterclockwise was especially important with the speedo spring in play.
I washed the glass with soapy water, coated the gasket with (Thanks Bill!) anhydrous lanolin, and completely wiped the excess lube before putting the gasket around the glass. Petroleum jelly works well in place of lanolin. I wore gloves during this process to not smudge the glass.
Then I fit the glass and gasket into the chrome. I gently maneuvered the case under the tabs in the chrome and used a pair of vice pliers to gently fold over most tabs. When everything was good, I tightened the brass nut.
I cut a new outer gasket (Thanks Steve at Moto Throwbacks!) from dense craft foam 5 mm thick. I installed the gasket from back to front, then put the speedo assembly in the housing from front to back.
I replaced the V-shaped locking piece and applied firm, even pressure as I positioned the washer and tightened the 17 mm nut. Even though I won't wire up the bulbs, I replaced the sockets to seal out moisture.
I trimmed the excess foam from around the housing and reinstalled the speedo on the bike. The 10 mm bolts mount directly on the fork.
One last thing: don't turn the front wheel backward too much. It's always better to make a U-turn. Fishing out a broken cable end is not fun.
Hope this helps. — pjc