Mobylette fine tuning

🇮🇹💦 Of the Loin /

I have a motobecane mobylette I think its a 77 that runs alright, its a little slow to accelerate but it does eventually do it(I do think its because im a big guy, but even on the flats the acceleration is a bit sluggish from 20-25)

the problem is it wont idle. It did yesterday after me and thor replaced the throttle cable, and today it wont do it. I found out the spark plug I was using wasnt burning hot enough (black carbon deposits all over it, made it not start) so I got a new plug, but according to the guy at kragen itll work but I need to get a BH5K NGK spark plug.

The carb is in decent shape (recently cleaned) and Im wondering if the spark plug is the cause of the idle problem (I can live with a sluggish acceleration, it is only a 50cc engine)

Re: Mobylette fine tuning

If it did idle yesterday after messing with the carb, it's capable of idling.

you dont say what plug you're using. The BH5K is "hotter" than the BH6K, but no plug will keep itself clean if it never sees top speed in a hot engine, nor if the engine is always running rich or cold for some reason.

At lower speeds plugs collect unburned fuel deposits.

As speed increases a carb delivers a leaner and leaner mix. Near top speed the combustion chamber and the plug is running nice and hot on a lean mix.. the plug insulator gets red hot. This is where the plug cleans itself.

A too-cold plug could get fouled to the point where you get misfires after only a few miles or riding at below top speed but that's pretty extreme. If it did, I'd suspect something other than the plug heat-range was to blame.... late ignition timing, a rich-carburetion problem, oil problems, crappy-dirty or old fuel.. a clogged air intake filter or air box restriction, a partially closed choke and similar.

Being able to set a good idle is an almost automatic reward for getting everything else correct first..

Re: Mobylette fine tuning

🇮🇹💦 Of the Loin /

the plug that came with it was an ND15FS I think. It was some wierd japanese plug that the guy at kragen looked up, and a cross reference for NGK showed a BH5K (which they didnt have, although they have the 7K for my garelli)

I had the moby up on the kickstand and revved it at full throttle after I cleaned the plug when I noticed the deposits for a good 2 minutes, and some revving up and down for a couple more. It still wouldnt idle, and when I took the plug out it was still covered in black soot.

I notice when I drive it that the lights in the speedo fluctuate alot but they dont do it when its on the stand(does that mean anything?)

also I dont get any misfires so far, ive driven it about 10 miles perhaps

Re: Mobylette fine tuning

the black soot is unburned fuel.. but running on the stand is inconclusive.. the engine is not under much of a load just spinning a wheel.

the symptom could be due to a variety of things.. a high float level, a float needle that decided to leak a bit.. air intake restriction, late ignition timing, etc.

That flickering speedo light could be a clue. Although lights and ignition are supposed to be on completely separate circuits, they may not be. Aside from worn insulation, a loose ground or a crossed wire , high voltage spark energy may be leaking across somewhere or other.. and road vibration is influencing it. .. who knows..

but something's not right .. this is unnatural for a stock machine in well tuned condition. I think the short route to finding it is gonna be a thorough tuneup and inspection with an eye towards the unexpected.

Re: Mobylette fine tuning

🇮🇹💦 Of the Loin /

the lights only flicker when its under a load (running on the ground not the stand)

The new plug that Im using (not the correct BH5K replacement) seemed to have much less soot on it (almost none at all)

I dont exactly know how to tune up a moped but im sure that I will be able to figure it out. There is some rust on the bike so it could be some worn insulation or a loose ground.

If you have any ideas about where I should start or what I can do I would be very grateful, thanks.

Re: Mobylette fine tuning

Experience is the best teacher.

There are 3 moby manuals HERE break> Study them so you know your bike from one end to the other.

As far as the engine operation goes, search and read until you understand how the basic 2-stroke engine works break> A good general tuning resource is Fred's Guide at the top of the page where the manuals are.

With a few basic tools and some small amount of understanding you can fix and tune the bike .. a moped is as simple as a motor vehicle gets.

However, moped restoration is a serious hobby that requires study, time and effort at a minimal dollar cost. Nobody knows everything and the 2-stroke engine's overlapping functions can be a challenge for the best mechanical minds..

but once a bike is in good shape it's as fun and economical a ride as there is anywhere and is well worth the effort, imo.

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