First read Fred's guide it is a good resource, and for most mopeds it answers the questions you probably have. Motobecane mopeds are a little different, and Fred’s guide on checking, and setting timing does not really apply.
Before we begin
To start a Moby hold the decompression lever and pedal the bike (rolling or not). As soon as you see the speedometer lights shine (or are certain it is turning over) let off the decompression lever and give it a bit of throttle.
Things you should have on hand:
- Carb cleaner
- Terminal strip
- Emery boards (or fine sand paper)
- A few spark plugs (Bosch W8AC, Champion L86C, NGK B5HS, B6HS or B7HS)—gap should be .4 -.5mm (keep a new plug with you or in your storage compartment at all times, and the tool to change it)
- Small plastic ruler with millimeter measurements
- Cam/clutch puller (needed to reset timing) Moby specific 381 (PIC 1a), or 2 arm puller.
Checking for spark
Attach a new spark plug to the the plug cap, clamp the plug cap to the engine (PIC 2), and get in a dark place if possible. Now attempt to pedal start the moped while looking at the plug, you should see a nice blue spark. If you have spark, skip to the next section, if not check the following. Note: some mopeds need a good brake light bulb to run; Motobecane mopeds do not.
- Disconnect the wires that connect to each other near the ignition coil. Keep the wire connected to the coil, but disconnect the other wire that is attached. This will disable the kill switch which is often faulty. I recommend leaving it disconnected for these reasons:
- You will still be able to stop the engine using the decompression lever
- You take a very bad switch out of the circuit that can short out and cause problems
- You will never pedal around the block with the bike in the off position.
- The horn portion of the switch will still work
- Clean your points, to do this you will need to pull off the flywheel cover, which can be very difficult. With any luck your flywheel nut has not been completely stripped out. The flywheel cover is reverse thread—you will need a 3/8” ratchet, and probably a pipe to fit over it to get some good leverage (PIC 3). You will need to keep the engine “locked” to be able to unscrew the flywheel cap. The motobecane workshop manual specifies a piston stop, but I have had good luck using a second ratchet with a *mm ratchet, holding the clutch nut, and stopped against the pedal (PIC 4) If you still can not get it off, trying heating up the nut with a butane torch before attempting again. (a note, some people use heat, but below the cap is the coils that are VERY sensitive to heat and can short out if the winding melt so use heat at your discretion) Another easy way to remove a stripped Novi Cap nut is to take a cutting disk and cut a flat edge on eash side making it a rectangular shape, then removing with a adjustable wrench.
- Open and close the points. Does the point’s connection look solid? (PIC 5) Do the point surfaces look clean? Spray some carb cleaner on an emery board and place it between the points. Let the points close, and pull it through. Do this a few times for each side, then pull a dry clean piece of paper through. Points gap should be .35mm at max, resize if necessary using flat head screwdriver. Replace flywheel cover, and try to start.
- Remove spark plug cap from external coil and examine plug wire where it makes an electrical contact inside the coil, this connection is often bad. Is the wire corroded? Does there look like a chance the connection was not tight? Strip a half inch of the insulator off the spark plug wire, then cut the copper off so it extends ¼”, push the copper out to the sides, and push a small screw driver into the opening to create a small opening (PIC 6). Now carefully attach the spark plug wire to the coil, make sure the connection you just cleaned up fits snugly on the coils.
- Examine all wires, and terminations through out the moped, and inside the headlamp. Look for places the wire seems damaged or crimped, or the connections seem loose, or corroded paying close attention to the connecting block behind the engine. I recommend replacing the connection block with a new terminal strip (available from Radio Shack) (PIC 7). Cut off a 4 connection piece of the terminal strip, cut and strip the wires, and make nice new connections using the terminal strip.
- Note: The flywheel is held in place to the timing cam by two dimples. If at some point the Novi nut comes loose and lets the flywheel wobble on the cam those dimples lose the round that hold them in place making it very difficult to keep the flywheel attacked properly. When is does this the flywheel can either come off completely, smash into the coils, move the timing cam, or at least make a bunch of noise. It can be advised to apply a but of semi-permanent thread locker on the Novi nut when assembling. The motors are not perfect and either an improperly tightened nut or extreme over-revving (such as coasting down a long steep hill at full throttle, seen it happen a couple of times to other peoples bikes as well as my own) can make the whole assembly come apart. Consider yourself warned.
Checking the timing
You do not need to have the flywheel cover off to check the timing, but it will help you understand how it works to see the movement happening.
Pull the black, and brown wires off the stator plate, disconnect the grey wire that is connecting the stator plate with the spark plug, and the kill switch, connect a multimeter to the grey wire you just disconnected, and the other end to the engine ground. Set the multimeter to continuity, and then turn the engine over by hand. With every rotation the multimeter should change from open to closed circuit, you should see the points opening and closing as well. (PIC 8)
Note: On Mobys with 6v systems (2 coil), the point opening can be checked without removing the flywheel by testing continuity from the gray wire that goes to the HT coil to ground, on 12v systems (3 coil) the flywheel must be removed, the three wires attached to the points must be removed and kept from grounding (unscrew and place a piece of paper bwtween them and the points) place a lead one eash side of the points and test to see if the meter registers open and close before trying to set timing, the systems are wired differently but the other steps of the procedure are the same. 12v systems are on 78 and later models. (although the ignition is still 6v for both systems, the lighting is split with 12v going to the head and tail lights, and another 6v circuit for the brake light and horn)
With every rotation you make the piston goes up and down once. You want the piston to be 1.5-2mm before the top dead center (TDC) of the cylinder exactly when the points open. The AV89, Cady, Commuter, and MobyX models need 1.5mm. Models in the 7, 40, and 50 series need 2mm.
Remove the spark plug, and wedge the ruler in the engine heat sink near the spark plug hole, place, and hold an allen wrench so the long side is resting against the top of the cylinder. A peice of masking tape around the wrench allows you to mark easily with a pen TDC and 2mm before. A razor blade on the head fins against the wrench gives a nice level to measure the lines against as well. Now as you turn over the engine the allen wrench will go up and down with the cylinder, and the multimeter will be displaying the point’s status. (PIC 9) You want the points to open (become an open circuit) right when the allen wrench is 1.5-2mm before TDC. Special tools can be purchased for this very critical task that thread down into the spark plug hole and have graduated markings on them. 0.5mm is not very long, but at that close to TDC translates into many degrees on the crank. A graduated timing wheel is also recommended for more accurate results. All of this is up to your whim, plenty of people get on just fine with eyeballing the critical measurement of ignition timing with mopeds (on mobys, the two dots on the cam should be lined up with the piston axis when the piston is at TDC if you want to eye it btw). Do what you are comfortable with. Ignition timing is crutial to engine performance btw, If you are not inclined to agree, try setting your timing at say 1mm or 2.5 and riding it around.
Setting the timing for this you will need a cam puller (PICS 1a, 1b). Be sure you thread the cam puller all the way into the cam before you try to remove it, the threads in the cam can be pulled out if one is not careful. Remove the cam, and push it back onto the rod gently into the position you think is right. After doing this, re-check the timing again to see if it is right (it will probably take a few attempts to get it right). If you leave the cam on loosely with the meter still reading point opening, you can simply rotate the cam until exactly when it opens, assuming the cylinder is still at the proper TDC measurement, this cuts some of the guess work. Once you have it right, put a small piece off wood against the cam, and bang it firmly back on with a hammer, check the timing one more time, and make sure you replace the wiring that was detached if working on a 12v system before replacing the flywheel cover.
Viva La Resistance!