Help needed with motobecane

Hi all

can any one please advise me on problems that I am having with a Motobecane Mobylette AV76. using a Gurtner carburetor? Basically my problem is that the engine runs, but after a few minutes it slowly peaters out and stalls and then will not restart. The spark is good, the plug is not too sooty or wet when it stalls. fuel seems to flowing ok, the carb is clean. The bike will idle when first started. The air filter is clear. Usually once the bike has then stood for a while it will start again. One it has stopped the engine turns over ok, but will not fire. It is currently running on one piston ring until I can get another.

Also what is the best fuel/oil mix to run this bike on?

Re: Help needed with motobecane

Jon Dalton /

It might be because of the piston ring. I have a moped that needed new rings and before we changed them it would run for a while and then die when it warmed up. What happens if after it quits you try pedalling really fast with it on the stand?

It could also be running too rich. After it quits, is the spark plug black or gas-soaked?

I forget, is the AV76 the model that has no clutch, or a later one? I own an older Mobylette and also know somebody with some engine parts for them. I can ask him if he has any rings, if you need one.

Re: Help needed with motobecane

Matt Wilson /

Check your fuel flow. It sounds like you are running out of fuel. After the moped stalls, remove the spark plug, and squirt a teaspoon of gas in the cylinder. Replace the plug and try to start it. If it starts you know fuel flow is you problem.

"It is currently running on one piston ring until I can get another."

Get another ring asap.

Matt

Re: Help needed with motobecane

yeah, if you run on one ring too long i can guarantee that you will ruin the piston, cylinder or both.

Re: Help needed with motobecane

or it could be somethin like the coil.

Re: Help needed with motobecane

The problems seems to be the ignition. When the machine is warm it loses spark. where do I go next?

Re: Help needed with motobecane

Ron Brown /

Martin,

Try this.

Ron

If you have a vom, check the ignition components as follows:

Disconect the wires from the points. One of these wires goes to the magneto ignition power coil. You should read a low resistance to ground on this wire, 1-2 ohms.

Another wire goes to the condensor, read this to ground on RXMAX, that is your highest resistance range, the needle should kick, then settle down to infinity. Reverse the vom leads, same thing should happen.

Another wire goes to the ignition coil primary, this connection is also connected to the kill switch. Disconnect the kill switch wire only from the ignition coil, the wire to the coil should read 1-2 ohms to ground.

From the wire to the kill switch, to ground, you should read infinity with the switch in "run" and 0 ohms with the switch in "stop".

Connect the vom from ground to the wire terminal on the points and make sure they are 0 ohms when closed and infinity when open. This seems dumb, but sometimes points that look clean do not connect.

Re-attach all of the wires to the points. Disconnect the wire from the points at the ignition coil. Measure this with the vom, it should read near 0 ohms. If your meter is sensitive enough, you should see a slight deflection of the pointer as the points open and close.

With the vom still attached and set to about 12v DC range, spin the motor by hand. You should see some deflection of the meter needle.

Pull your plug (spark, that is) and make sure the piston is at or very near the top of it's stroke when the points open.

Re-connect the coil wire. Measure from the plug cap to ground, you should read 5-15k ohms. If this is infinity, unscrew the plug cap from the wire and read through it. This should be 5-15k ohms, if it is open, replace it, if greater than 15k ohms, it may still work, but replace it with a paper clip for test starting then re-install it after you get the ped running.

Read from the end of the plug wire to ground through the coil, you should see 1-5k ohms. If this is infinity, the coil is bad.

Lastly, using the ohmsX1 scale, verify that all of the grounds really are.

None of these tests are guaranteed to indicate a problem, especially where you read coil resistances and they seem ok. Substitution is the best test here.

To test the coil, connect a spark plug from the plug wire to coil ground, connect an ignition condensor (any will do for this test) between the points terminal and coil ground. Connect the negative of a 6V battery to coil ground and touch the points terminal momentarily with the positive lead. This should give you a spark at the plug.

Re: Help needed with motobecane

Check and see if your exhaust is clogged?

Walker

Re: Help needed with motobecane

martin,

motobecane usually suggest using 3 oz of oil per 1 gal. This is the correct mixture for late 70s models. You need to search the moped owners directory and try to find someone with the same bike, not just motobecane and ask what they use. prolly your best bet.

casey

Re: Help needed with motobecane

Ok

I have found and fitted another pair of rings.

I have warmed the machine up until it fails, tested the spark and I have a good spark at this time, squirted fuel in the cylinder but does not restart until it has cooled a little. Exhaust is now clean.

What next?

Note this machine does not have a kill switch, it stops by decompressing.

Thanks to all those who have tried to help so far.

Re: Help needed with motobecane

Martin,

are you totally sure that there is no kill switch? I thought decompression whas the kill switch on my Mobylette 50 series... but in actuallity, its in the horn switch... some kind of combo, but it didnt have any markings to tell. I dont know much about your bike, but i wasted a good bit of time dicking with this simple mistake. Anyways, that shouldnt matter much, b/c it is running.... but are you sure you have the correct plug in? When i bought my motobecane, it had a plug in it, similar to a lawn mower plug... when i was out buying new ones, i based what i bought off this plug.... you have no idea how pissed i was when i found out i had the wrong plug! Check internet and see if you can find what motobecane suggests. Then youre gonna have deal with the autoparts stores to make sure they dont give you something wrong.... they are real pricks about this kind of stuff. I had some asshole tell me that my indian didnt exist, and it was a fake.... even when i had it in the parking lot! Just some advice, Something simple like that took me a while to notice... and alot of pedaling to find out!

Good Luck,

Casey

Re: Help needed with motobecane

Ron Brown /

Martin,

The most logical scenario is that you are running too rich.

What does your plug look like.

Ron

Re: Help needed with motobecane

The mix is Ok, I am running at 25:1.

I have a new condenser on its way to me, as I have been advised that this could give problems under heat and load. Since it was a $10 option to try, I am going to eliminate that one first. Then the coil is next!

Thanks again, I will let you know how it works out.

When it is running it is nice to ride. It was bought with some cyling motor pacing work in mind, but is probably underpowered. If I get it running well, I will up the pedal assist gearing by brazing in a second bearing housing and double staging the gearing, this will allow the pacer to assist with the speed and vary it sublty so keeping the cyclist flat out. That is the plan any way!

Re: Help needed with motobecane

remember that gurtner carb is restricted... it limits your fuel flow so you can only travel at a limited speed... check out a replacement solution, bigger jets, or maybe a new muffler before mechanically modifing the bike.

Re: Help needed with motobecane

Ron Brown /

Martin,

OK. I admit it. I'm confused.

You post about how you have a good spark, then in your next post, you are buying a new condensor. It is true that the condensor can give problems under heat and load, but if you have a good spark, you have a good condensor and coil.

What color is your spark plug?

25:1 seems like a lot of oil. Where did you get that ratio?

Ron

Re: Help needed with motobecane

Why a new condensor? I do get a good spark, out side the engine, however under "load" inside the cyclinder can produce different results. So for the sake of a few $ i can eliminate bone possibility. If that fails then I look again.

The plug is OK is colour.

The 25:1 ratio should be OK for this engine, it is one of the earlier ones. C 1962.

Gearing up the "pedal" side to improve speed. This should not really cause mechanical problems. All I am doing is allowing an extra engine (the rider) to add to the power. If i am riding up hill, it is ok to pedal assist the engine, and possible because I am going slow enough to allow my pedal cadence to keep up using the small pedaling gear.

By increasing the pedal gearing, (I have to double stage it as there is no room for a single stage, I will probably use 36X12 driving 38X16), I will be able to pedal fast enough to assist the engine. I only need to increase the speed by about 4 mph. If this causes the engine to rev out, I can always gear up the engine side.

Re: Help needed with motobecane

Ron Brown /

Martin wrote:

>

> Why a new condensor? I do get a good spark, out side the

> engine, however under "load" inside the cyclinder can produce

> different results. So for the sake of a few $ i can eliminate

> bone possibility. If that fails then I look again.

Martin,

What you say is true, however, unless the problem is in the high voltage side of the ignition circuit, a good spark outside the cylinder will give a good spark inside the cylinder. Possible high voltage culprits are the spark plug, plug cap, wire and coil. Everything else is low voltage and not affected by "load".

This "load" is increased resistance accross the plug points because of the increased resistance of the compressed mixture. If you think this may be a problem, you can simulate a plug under "load" by opening the gap on an old plug and using it to test with. Of course, this does not test the original plug.

As long as your ped runs ok cold, you may want to test the spark voltage with a large gap when cold, then compare it to a hot spark.

>

> The plug is OK is colour.

>

> The 25:1 ratio should be OK for this engine, it is one of the

> earlier ones. C 1962.

>

Sorry, didn't realize it was that old.

> Gearing up the "pedal" side to improve speed. This should not

> really cause mechanical problems. All I am doing is allowing

> an extra engine (the rider) to add to the power. If i am

> riding up hill, it is ok to pedal assist the engine, and

> possible because I am going slow enough to allow my pedal

> cadence to keep up using the small pedaling gear.

>

> By increasing the pedal gearing, (I have to double stage it

> as there is no room for a single stage, I will probably use

> 36X12 driving 38X16), I will be able to pedal fast enough to

> assist the engine. I only need to increase the speed by about

> 4 mph. If this causes the engine to rev out, I can always

> gear up the engine side.

I like all these gearing ideas, this is whay motorcycling used to be about.

Ron

Re: Help needed with motobecane

david f martin /

Looking at pics... great bike! Great thread, too.

I'm an aircooled VW (as in, "antique") mechanic, now a scooter mechanic, too. I love this stuff...

Working on old stuff is a lot different from working on new stuff...

david

Re: Help needed with motobecane

Hi

Would like to see what an Motobecane av76 looks like. Is there a photo somewhere? Thanks John

Re: Help needed with motobecane

When the bike is back togeather and running I will do some photos for any one who wants to see.

I will first get the machine road legal and through its "MOT" (British certificate of road worthy ness) before I do the two stage modification of the gearing. When the time comes to re test it (MOTs are done anually) I will simply remove the second stage chainwheel and replace the original chain, then all the modification will be hidden under the side cover.

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