76' Motobecane Mobylette overheating

Hi, i have a 76' Motobecane Mobylette that was given to me (because it has problems) and its now my job to fix these problems.... more specifically it overheats after about 10 minutes and shuts itself off. water sizzles off of it. also a little bit of gas slowly drips out of it right where the motor meets the exhaust. my uncle thinks its a problem with compression... the guy who gave it to me thinks its a problem with the "magnatron"... which ive never even heard of... i dont know much about engines... this moped was jsut given to me yesterday.. i had never even heard of a moped before then... but i do have a bit of common sense so if anyone has any help to offer it would be greatly appreciated.. thanks!

Re: 76' Motobecane Mobylette overheating

He meant magneto, not magnetron. A magnetron is an electronic device that emits microwaves. And unless the timing is very badly off, I doubt the magneto is the problem.

First and foremost, you need to make sure that it has the correct spark plug. On an air-cooled engine, the spark plug acts like a kind of thermostat. Too hot of a plug will overheat the engine.

And I'm not totally sure that the engine is actually overheating. Water will boil off of any fully hot air-cooled moped engine.

When it shuts down, does it seem like the engine is locking up? If you try to restart it, does the engine rotate freely, even if it does not start?

Re: 76' Motobecane Mobylette overheating

it still rotates.. and it tries to start.. then backfires loudly. you sure water is supposed to sizzle off of it? that sponds kinda dangerous if a kid was using it.. as for the spark plug.. i have no idea, i got it yesterday.. the guy was "restoring it" and i dont think he had much idea what he was doing.. i have a bunch of yamaha motorcycle/snowmobile sparkplugs.. would those work? and also.. is it supposed to have an electric start? theres an ignition knob on it that does nothing.. and if you push it its a horn.. although when you turn it it clicks.. and when you let go it clicks itself back

Re: 76' Motobecane Mobylette overheating

go HERE and grab all the motobecane manuals.. get to know the bike ..

Aslo visit the 'Resources' section up top of this page. Read FREDs GUIDE break>

"Search" (link's up there somewhere) this forum for specific questions that come up..

Re: 76' Motobecane Mobylette overheating

I guess it depends what part of the engine we're talking about, but the cylinder head and cylinder can easily get to well over 300 degrees F. The exhaust pipe will give you a 3rd degree burn in an instant. Of course it's dangerous, but you're not supposed to touch it when it's hot..

That knob is the combination horn & ignition kill switch. Push for horn, twist to kill the engine. It snaps back so that you can't accidentally leave it in the 'off' position.

It does not have a battery, or an electric starter.

Correct plug should be an NGK B5HS or Bosch W8AC. Autozone sells the Bosch plug.

Get the correct plug first, then decide if anything is really broken or not.

Sometimes the ignition condenser has a thermal failure mode. It works fine when the engine is cold, but fails when it gets not (but not necessarily overheats). It would be one of several things to investigate if the proper plug doesn't fix things.

Re: 76' Motobecane Mobylette overheating

thanks, youre very helpful. ill try to get that plug today or tomorrow.. and ill post again when i do... thanks again.. one other thing... it has another kill switch on the throttle handle.. why have 2 kill switches? (ive been using the one on the throttle)

Re: 76' Motobecane Mobylette overheating

That's not a kill switch on the throttle handle, it's the decompressor lever used for starting the bike. Some people do use it to kill the engine, but it's not the primary intention.

Follow those links that Joe gave you, and RTFM. (Read The Fine Manual).

Re: 76' Motobecane Mobylette overheating

manuals.. long.. tedious....strenuous... i think ill just try the spark plug first.. then ill read the "fine" manual

Re: 76' Motobecane Mobylette overheating

It probably is not overheating

I would check things in this order

In fact this is what I do with all the mobies I find

-clear gas cap vent holes

-replace plug with new champion l86c or equivilent

-Put in new gas with good oil and 40:1 mix

-clean carb well (pointy side up on float)

-examine, restrip sparkplug wire on both ends

-replace electrical connection block near engine

-remove the kill switch from circuit by disconnecting grey staycon connectors near high tension coil

-check timing (if it is overheating, it is probably this, flywheel cover is reverse thread))


-replace condenser.

Re: 76' Motobecane Mobylette overheating

ill have to have my dad do most of that.. im 17 and a complete newbie.. i have no clue about anything cept the spark plug.. which will be the first thing i try.. thanks!

Re: 76' Motobecane Mobylette overheating

ok, replaced the plug, cleaned the carb, put an air filter in... now it lasts a little longer, but it still eventually overheats and stalls, but it only stalls while i idle.. so as long as i keep it running, it stays on.. but as soon as its off, it stays off for at least 30 mins... actually, eventually it stalls even when im still giving it gas.. oil is mixed properly... the only other thing could be timing, eh? that sounds really complicated... sigh why cant anything ever be simple... cries

Re: 76' Motobecane Mobylette overheating

are you sure your getting a good gas flow?

Re: 76' Motobecane Mobylette overheating

when it stops running, immediatly turn the fuel valve to the off position.

Now check to see if there is any fuel in the bowl.

if there is fuel, then I suspect the condenser. If no fuel, then a flow problem. check the gas cap vent hole, and remove the fuel petcock to look for blockage.

put in an inline fuel filter (for lwanmowers), and clear fuel line if you have not allready. it can be very helpfull to see gas moving.

Allthough setting the timing can be a little tricky, testing it is easy, read the guide I pointed you toward earlier.

Re: 76' Motobecane Mobylette overheating

Agreed.. this bike doesn't really sound like it's overheating. Just failing when it's hot.

Keep looking for things in the ignition or fuel system.

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