Motobecane 50V - No Spark

I recently purchased a 1977 Motobecane 50V, and I am working to get it running by spring. Currently, it will not start at all. I am a newbie to moped repair, so please bear with me if I have failed to understand something fundamental.

The first thing I have tested for is the spark by following Fred's Guide. There is no spark. The one thing I have noticed is when I turn the pedals, the belt rotates the variator smoothly. However, the magneto only occasionally "catches" and turns. I usually have to pedal a little bit faster to get the magneto to turn, and it will only turn about once for every 4 or 5 rotations of the pedals. By listening carefully, I also believe that the piston is only moving when the magneto turns, but I could be wrong about this. Should the magneto be rotating at the same rate as the variator and pedals?

If this is even possible, it feels like the magento is somehow loose, such that it only occasionally "catches" and turns. Can anyone point me in a direction I could explore?

Thanks in advance for any advice!

-Aadm

Re: Motobecane 50V - No Spark

Your moped should have a decompression lever on the right handle bar. It runs a cable to the cylinder head and opens a valve in the combustion chamber making it easier to turn the engine over.

However, this shouldn't be necessary if the spark plug is removed. Did you remove the spark plug prior to checking for spark, or did you maybe just use another one, or are you just holding the cap near the engine?

Try turning the engine over by hand. Try and figure out if there is resistance to turning over, and if it's cause by compression or by something like rust or corrosion in the cylinder or bottom end.

Re: Motobecane 50V - No Spark

Bret, I removed the old spark plug from the engine, and then attached a new spark plug to the cap and held it near the engine block to test for a spark. So, it is not the compression that is preventing the engine/magneto from turning, because the plug was removed at the time.

It seems plausible that there may be rust or corrosion in the cylinder bottom. Wow, it sounds like I may have have to disassemble the engine. That will be a new one for me. :-)

-Adam

Re: Motobecane 50V - No Spark

See Ya Moped Army /

Does the flywheel turn by hand? If so, the piston is not stuck. What you described in your first post sounded normal to me. If you can rotate your flywheel by hand....remove the flywheel using a 3/8" socket drive and turn the nut clockwise (reverse thread). Use a nail file and clean the points real well and replace the flywheel. Rotate the flywheel by hand and see if the spark plug sparks when held on the cylinder head. 99.99% of the time, it's dirty points that keeps them from sparking.

Re: Motobecane 50V - No Spark

the way your magnetto is turning sounds correct, you have to pedal pretty fast to get them going sometime, yes that is the only time the piston is moving.

read this

https://www.mopedarmy.com/forums/discuss/1/173041/173041/

Re: Motobecane 50V - No Spark

For a novice, I reccomend you get someone who knows more about machines and engines to try and determine whether your engine is rusted up. If this is thecase, it would be easier and cheaper to get another engine, or at least another bottom end, rather than rebuilding.

Also, the magneto is the drum on the right side. The left side holds your variator and clutch mechanism. When the belt moves the inner clutch drum fast enough, it grabs the outer clutch drum, which turns over the engine. Usually with the combustion chamber open this is pretty easy. It's possible that your belt or clutch is slipping.

If the bike hasn't been run in a long time, i would out a small amount of two stroke in the spark plug hole, and in the intake if possible.

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