79 Moby- won't idle

Mr. Richard Dickrich /

Hello,

I just purchased a 79 Moby and it just won't idle. It starts very easy, and gets up to 20 mph pretty consistently, but feels like it could give a little more at the 20 mph top speed. It is a type A so it should go 30mph. It will run without the throttle for about 5 seconds and the stalls when the bike is about to stop moving.

The bike is in tip top shape with only 300 miles on it - Like new condition (odometer works). Carb is like new, airbox is super clean, throttle works great. I adjusted the throttle screw and nothing happened- Does it ever? not in my experience.

I've read Fred's guide many times and this thing is in such good shape, I don't want to start dismantling it. I was afraid to even touch the carb.

Anyone have a similar problem they can advise on to get this thing idling- topping out? Thanks for the help.

Rich

Re: 79 Moby- won't idle

Well, I just want to say, I have the same problem, my '79 50V wont idle. Im pretty sure a dirty carb could cause this, so thats one possibility. Another is that the clutch isnt disengaging. So, anyway, Im still trying to fix this on my Moby.

Re: 79 Moby- won't idle

did you adjust the screw where the cable goes into the carb? mine did the same thing and i gave that screw a little turn and it worked. also... there is a needle screw on the side of the carb. that is the one where you can control the idle. if it wont idle turn the screw on the side of the carb 1/2 turn out/to the left. this should work!!!

Good luck!!

Kev

Re: 79 Moby- won't idle

here is a pic

1152624364_gurtnercarbmm.jpg

Re: 79 Moby- won't idle

mopedwhisperer /

When you cleaned the carb did you take it ALL the way apart..? That little diffuser (brass thing) up inside that passage(small flat head screwdriver ,I think... been awhile) needs to be taken out and physically cleaned with a single wire brush quill...carb cleaner and compressed air can`t quite get it...that is the idle circuit !!! If the bike still will not idle could be those feeble crank seals that motobecanes have they seem not to survive the 25+ years of sitting like some other mopeds do...that could explain the the lack off top speed ...hope this helps

Re: 79 Moby- won't idle

I had this same problem with my yardman. When i was slowing down i had to give it a little gas right before stopping or it would stall. took a bit to get used to but once you get the feel its all good. The guy i bought it from said that it was how you downshifted or something like that. If its not your carb id say thats it.

Re: 79 Moby- won't idle

Ben Van Zoest /

Retorque the head bolts any backchamber leak causes top speed loss and idle problems, false air. The trottle cable may have streched to the point were the air valve is almost closed.

Re: 79 Moby- won't idle

there's no reason to be afraid of touching the carb..

A carb's outside appearance reveals nothing of importance. Gasoline deteriorates in time and the residue partially plugs up tiny internal passages.

Everything points to a lack of fuel delivery so just clean it.. In a day or so it'll idle nice and you'll be goin 30mph.

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

Sorry for the confusion. I meant to say I was ALMOST afraid to touch the carb.

I did take the whole carb apart an it was sparking: jet, float, bowl. All clean and like new. I've done this on a few bikes so I am fairly educated in carb cleaning.

I'll try the ideas so far, thanks for the help and keep them coming. I'll report back. This thing is a looker, trying to make it a runner...

Rich

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

Thanks Whisperer. I did take the diffuser off, it was spotless.

This is dumb, but where should I check the crank seals? Would a compression test tell me if they were working OK?

Re: 79 Moby- won't idle

one question. Does the cold engine idle?

A crankcase compression test will reveal bad seals. The crankcase should hold 6 psi for a few minutes .. use a hand pump and a low range pressure gauge for accuracy.

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

Thanks Kev. I think your cirlced picture is of the idle screw and I have tried that.

Can you give a little more detail on that first screw that you are talking about? Are you taking about screw 14 in your picture?

Rich

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

Thanks Chris. Is there a visible way to tell if the clutch is not disengaging?

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

Bernard- Will a compression test answer if the re-torque is needed? I'll try it as well.

How can you tell if the air valve is closed. I took the carb apart and the throttle was not blocking all fuel/air flow to the engine. There was a gap .. Is that what you are talking about? Thanks.

Re: 79 Moby- won't idle

just to be sure you know where the whole idle/enrichment circuit is, i've attached an image.

It's easy to miss part of it. Having a clean diffuser is a good start.

1152643642_gurtner_enrichment_circuit.jpg

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

Oh sweet, thanks for the graphic. I think I was confused. I will check that again today for sure.

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

No, it does not idle when engine is warm or cold. I'll check crank case this afternoon if I can figure it out. Thanks.

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

OK, the engine definitely stops running as soon as the bike stops moving. I can drift down a hill with no throttle at all and the bike will remain running. As soon as it drifts to a stop, it will stall unless I give it gas. Does that point to the clutch?

The carb is spotless, diffuser and all.

I need to check the crank seals but don't know how to do that.

Thanks All!

Re: 79 Moby- won't idle

have you checked for vacuum leaks?

A good idle says everything is perfect .. almost any little thing can kill idle.

Last i heard your top speed was suffering badly.

If that situation hasn't changed and the bike runs ok up to it's current top speed, the trouble still points to carburation.. air or fuel .. too much or too little of either.

:

seal testing..

Block off all ports with metal plates and rubber gaskets.

One of the "gaskets" can be the part of an inner tube with the valve stem. That's your valve to pump it up.

Stick a low-pressure gauge, like 0 - 30 PSI somewhere.

pump it a bit with a hand bike pump .. spray soapy water eerywhere. Find and fix any air leaks.

then pump to 6psi, watch the gauge and wait ..

btw..

the reason i asked earlier if it idled cold was that If it idles cold, but not hot, it might be a choke thing.

a closed choke will allow cold idle, kill hot idle, and limit top speed.

Re: btw..

Mr. Richard Dickrich /

Thanks for all the help Joew. I think the crank seal test is too advanced for me at this point. I'll go there next, but I do think you are right about the air/fuel mixture.

I have a couple more details to share- my choke does not kill the engine when it is running. That led me to believe that I was getting too much air. So, I taped off some of the airbox, and then my choke would kill the engine when running, but my top speed was only 10MPH! Can't figure that one out... any ideas?

The Moby still goes about 20 MPH and does, for a lack of a better term, "fart" at full throttle instead of providing power- again ...making me think too much air.

That being said, the bike only has 340 miles on it. The manual says it will not yield full power until 600 miles. But I doubt it is a drastic change at 600.

I'd appreciate anyone's thoughts.

Rich

Re: btw..

if a closed choke doesn't kill the engine, it says that although air is cut off, the air:fuel _ratio_ is still close enough to being correct to allow the engine to continue to run .. with very little air.. at some RPM.

Follow that logic, and the conclusion is that there's very little fuel being allowed into the engine. That slight amount of fuel requires a commesurately small amount of air.

Air:fuel ratio means everything .. get ratio right and the engine runs fine.

Since the ratio must change according to engine load, etc, if either air or fuel is in limited supply, that will also limit the available ratio. And that ratio, lean or rich, will determine where the engine runs good, poorly, and not at all.

Bottom line is it seems fuel supply is being restricted.

:

Engine bearing breakin takes only a few miles ..

Rings are broken in when the cylinder's honing marks (rough crosshatched scratches left specifically for this purpose) have worn against the new rings, re-shaped them to the shape of the cylinder and have just about disappeared.

so look for crosshatching marks on the cylinder wall .. if they are gone, breakin is over.

Re: btw..

Mr. Richard Dickrich /

Thanks again for all the help. I will focus on the ratio. Since the bike was going 20, I assumed it was getting plenty of fuel. But I guess if the bike should go 30-35, that is 33-50% less than the top speed. I do have a couple of quick questions.

1) In the picture, does the throttle look too far closed? This is it's idling position. I assume that the gap to allow the ratio into the engine is designed to work best when the throttle is fully closed?

2) In the picture, the fuel line is attached to the left. What purpose does the open pipe sticking up over the float bowl serve? Is this just a vent for flooding fuel? It has not flooded for me. I have not seen other carbs like this. I was thinking it could be a component to a vacuum.

Thanks again. I think I am going to soak the carb again. ugh.

1152898020_mopeds_024.jpg

Re: 79 Moby- won't idle

Mr. Richard Dickrich /

It turns out that there was some blockage in my fuel bowl- part of the idle circuit. A little white ball of gunk It is running much better now. I will need time to fine tune the idle right, but it is running at much lower speeds and a bit faster top speed- I think my speedometer is a bit off. Thanks for everyone's help- you all pointed to it- dirty carb. So embarrassing.

Thanks all.

Re: 79 Moby- won't idle

everybody can admit the same thing has happened to them .. or else they're just lyin' mangy dogs..

Re: 79 Moby- won't idle

The worst is when you've worked on your bike for a while and can't figure out what's wrong with it, then you realize the petcock or kill switch is in the off position.

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