Vacuum Leak

Hello, I was wondering if anyone could help me out. I recently took apart the carburetor to clean it. I put it back in and my moped wouldn't start. It's getting gas and it'll start sometimes and run just fine for a little while, then sputter like it's running out of gas and then die. I'm almost 100% sure it's a vacuum leak, I'm just not sure where. Does anyone have any advice? Thanks a lot.

Re: Vacuum Leak

spray some Starter Fluid around the intake manifold area (not into the carb's intake) If engine rpm increases theres a leak.

And if some Starter Fluid in the carb's intake gets the bike started instantly every time it says that the carb is still a bit dirty or clogged or is not able to deliver fuel for some reason.

The bike could very well be running out of gas.. check fuel flow from the tank to the carb .. fuel should flow in a thick steady stream from the carb's fuel line. If a restricted trickle of fuel into the carb is not fast enough to refill the bowl, the engine will die.

also remove the tank's cap .. if fuel flows better and if the engine no longer dies the cap's air vent's probably clogged. A vaccum builds in the tank if its not vented.

Check float level.. if it's low the bike will run poorly and may cough and sputter as it needs more fuel than the low level can provide.

Re: Vacuum Leak

Don Pflueger /

check the gascap vent and petcock and any fuel filters.

Re: Vacuum Leak

Yeah, there is a strong stream of fuel when I pull the line out from the carb banjo, and when I take the float bowl cover off, the bowl is full of gas and the float is floating at the top. I've cleaned the inside of the carb a couple of times with both compressed air and spray carb cleaner stuff. I know the carb is clean, that's not the problem. I've put new gaskets in between the crankcase-reed valve and reedvalve-intake manifold and coated those gaskets with silicone sealant stuff. It's definitely running out of gas, like I'll kickstart it and it'll go, and it won't die when the throttle is on like full or half, but if I let go of the throttle for even a couple of seconds it dies. also, at full throttle, it doesn't fire everytime, it'll fire maybe 10-15 revolutions and then skip a couple, like 5 or so, and then another short burst and then no firing. You know what I mean? It sounds and behaves exactly like it does when it actually runs out of gas. And spark isn't the problem. I've tested this numerous times, and everytime it has very strong spark. I was wondering how often there are leaks at the crankshaft seals, you know, like behind the clutch or magneto, or if this is something that can actually happen. I tried to test for this, too. I took off the reed valve and placed a piece of plastic torn from a grocery bag airtight over the crankcase inlet. I then rotated the flywheel. It looked like there was a vacuum, like it tried to suck the piece of plastic in, but it wouldn't stay sucked in, it would suck in and then quickly return to it's original form. Does that make sense? I don't know what that means if anything. I really have no idea; but if anyone else does, I'd be very appreciative. Oh, and sorry about the longwindedness.

Re: Vacuum Leak

The crankcase is not perfectly sealed on the assembled engine. Covering the reed intake opening in not enough.

If the piston is high in the bore, air can flow up the side of the piston (there's some clearance here) and out the exhaust port.

If the piston is low in the bore, the piston rings do not seal unless very high pressure is pushing them against their seats and against the cylinder wall... and there's a ring gap. So the test you did is not conclusive.


The symptom does mimic running out of gas .. you described it well.

But what's up with this? :

_but if I let go of the throttle for even a couple of seconds it dies_

Do you mean it won't idle? Or do you mean the engine is dying when the bike is moving along at some speed when the throttle closes?

I wouldn't be quick to rule out ignition or carb trouble just yet, mainly because it closes the mind .. and there's no room for making assumptions. Main seals do blow but the symptoms don't suggest it, imo.

I won't list everything under the sun that should be doublechecked and done .. things like installing a new spark plug, checking ignition timing, etc.... all the normal tuneup things. Can we assume that all the troubleshooting hints in Freds Guide have been done and that there are no wires hanging in mid air and that this bike is in generally good shape?

Re: Vacuum Leak

and i was taking a peek at the Batavus repair manual .. that Bing carb has a weird little pivoted choke plate .. im not exactly sure how it operates but it's got a return spring and a cable .. Is this an automatic choke that is linked to throttle operation?

Check the choke's operation and adjustment .. i'm thinking that there is something going on with that throttle .. if closing also the throttle closes the choke somethings not right... a closed choke can make the engine die.

Does the choke open wide when it's supposed to? and does it stay open when it should?

Re: Vacuum Leak

Yeah, by the way, thanks so much for all of your help so far, I really appreciate it. Yes everything in fred's guide has been done. It has a new spark plug, I've checked the ignition timing. The bike is in pretty good shape with the exception that it won't run well; it only has about 3500 miles on it. There are two completely different cables, one for choke and one for throttle. the operation of one does not affect the operation of the other. I am _absolutely_ sure the choke at least is functioning exactly as it should.

when I have it started on the stand it will go full throttle after much sputtering; but once it gets to full throttle it will run alright _at full throttle_. it dies if the throttle closes if the bike is stopped or moving along at a good speed; whenever it happens, the engine dies if the throttle closes.

Re: Vacuum Leak

something aint right .. although i'm very familiar with my carb and could probably assemble it in the dark, first thing i'd be inclined to do is take the thing apart and reassemble it again..

the bike runs alright at higher speeds and that says two things.. ignition and compression are ok. The only reasonable thing left is carburetion.

Did you use any gasket compounds on the carb itself when you reassembled the carb? Did you do anything at all out of the ordinary or are any carb gaskets or parts altered in any way?

btw, when exactly did this problem appear?

Re: Vacuum Leak

See, it all happened about two weeks ago. I drove it to work and it ran PERFECTLY, then, I got off work and it wouldn't start. I took it home and did a bunch of diagnostic stuff to it. I went through fred's guide, filed the points, etc. when it still wouldn't start, we took the carb apart and cleaned it and stuck it back on. When it still didn't start, I went back to ignition and eventually figured out the condensor was failed, so I replaced it, and got good spark; but it still wouldn't start. See, the original problem was a failed condensor, and I think I created this carb problem, whatever it is.

Since I replaced the condensor, I've taken the carb apart a good ten times and checked it out. There's nothing in there that's obviously wrong. I'm still using the original reed valve gasket, though I coated it with silicone. I made a new intake manifold gasket out of rubber fiber gasket material, and coated that, too. When I took the carb apart the first time, there was no gasket between the intake manifold and the reed valve, there was just a thin layer of this rubbery stuff, which I imagine was some sort of sealant at one point in time. Originally I just figured this was the original gasket, but have since learned that there was supposed to be an actual gasket there, so I put one in.

I should also say that about ten minutes ago I went out again and it started after about ten kicks, and after about 5 more (see, it would go for a couple revs and then die) I got it up to full throttle. I rode it around the block a couple of times. I guess after it's been at full throttle for a little while, it'll idle for about 10-20 seconds before it dies, but it still dies. Then, after it died, it wouldn't start again, I tried to start it about 25 times, but no go, it didn't fire once, so, sometimes it runs, and sometimes it just doesn't. That's about all I can say about it right now.

Re: Vacuum Leak

lets say it's running out of fuel .. What could cause this if fuel flow to the carb is good and the carb is clean?

One thing is a sticking needle valve. It may not stick all the time. But when it sticks the float drops down but the needle doesn't open it's valve.

Try two things..

First, run the bike and cause the problem to happen.. when the engine dies immediately turn off the petcock and then remove the bowl.. see if fuel level is normal.

Second, make the engine die or wait till it dies and use a squirt of Starter Fluid in the carb intake. See if it immediately restarts without fail or still refuses to start.

Re: Vacuum Leak

Thanks, I'm at work now, but I'll try both of those things tonight and get back to you.

Re: Vacuum Leak

ok.. and do get some real Starter Fluid.. it's very volatile as compared to carb cleaner..

It's made for starting cold engines.. mostly ether but it has oil mixed in.. a little bit wont hurt the engine but the price is kinda stiff, like $5 a can or so.

Another thing that can prevent gas getting in is a hung-up float. On some carbs the float bowl can be reassembled wrongly and interferes with free movement of the float. A bent float hinge pin might do it too..

Re: Vacuum Leak

Okay, so tonight, after work, I took apart the carb _one more time_, and cleaned everything again, I checked and rechecked everything. I did everything I could possibly think of, and once I was certain the carb was functioning correctly, I put it back on. Surely it would start this time, I thought. So I tried to kick start it, but it didn't start. Rather than assuming it was the carb, like I have been doing, I assumed it was something else, as I was (and still am) certain that the carb is working correctly. So, I checked the spark and lo and behold, there wasn't any. Honest to god, the last time I checked the spark, about a week ago before I started all of this carb work, it was strong, solid, it was the best damned spark anyone could ask for. Sure, there is now some spark, every once in a while, but it's very intermittent, which, I guess, would explain the "running out of gas" behavior, because it would spark for about ten seconds and then not spark for five, which would mimic getting gas for ten seconds and none for 5. So, I'm back to square one. I'm pretty sure it's not the condensor, at least, as it is brand new, so that narrows it down to only about ten million other things it could be. Oh, and the spark plug is brand new, too, like, out of the box after I noticed it wasn't sparking, so it isn't that. The timing is right, too, because I checked that to some accuracy a couple of days ago, and I can't imagine how drastically that can change in two days. I was wondering if it's possible to find the short in the circuit with a voltmeter, because I've been thinking about getting one and learning how to use it. That would make it really easy, I don't know, if anyone has any suggestions, I'd be happy to hear them. Thanks.

ignition circuit

I recommend you start with an analog (not digital) multimeter.. might find a cheap little one for $10 or less.

Set it to the voltage function Hook up the voltmeter's red lead to the 'hot' wire of the kill switch. This wire goes to the points. The switch grounds the wire, grounds the ignition and kills spark..

Ground the meter's negative lead to the frame or engine block. Set the voltage range high as possible.

Start the bike and watch the meter.. it should read zero volts unless the Kill switch is activated. Then the voltage should spike.

An intermittant short to ground would make the meter's needle jump around.. This means some wire is grounding when it shouldn't.

Another possibility is the kill switch itself is bad..

Another is a loose or corroded connection anywhere in the ignition circuit.

_it ran PERFECTLY, then, I got off work and it wouldn’t start_

Could someone mess with the bike while you were at work? any missing screws or notice anything odd around the ignition key switch or anything weird anywhere on the bike?

Re: ignition circuit

I would really like to do that; but you see, I'm afraid that my particular ped doesn't have a kill switch. It is supposed to have one, I can see the markings on the handlebar where there once was one; but there isn't one any longer. I only recently bought the ped and have no idea what happened to it I got one off ebay that I was hoping to install just as soon as I had a running engine to kill, if you know what I mean. I really don't think anyone did anything to it at work, I mean, I didn't notice anything out of the ordinary, nor have I since. It would've been possible, yes, it was parked on the street, but I just don't think it happened. I think the condensor that I ended up replacing just spontaneously quit, if such a thing can happen. Right after I replaced the condensor is when it wouldn't idle. I was wondering if it was possible to check the voltage (or ohms or whatever) in different parts of the circuit to try and isolate which portion of the circuit is shorting; I'm afraid I really don't understand too much about electrical theory.

screwed up anyway..

its just as well that you don't have a meter... I'm pretty sure i screwed that up royally .. i was thinking ohms and saying voltage but it is still wrong. I can't decide if an ohm meter's fuse would be blown if the points grounded through the meter _OR_ if the engine would run at all if the meter was set to read volts.. Forget about those instructions.. i take it all back.

But do get a multimeter and learn the basics. It will come in handy.


So there's no kill switch? Where are it's wires?

In particular, are those wires' ends taped up and insulated and secure or are they bare and laying near a metal part of the frame and intermittantly grounding the ignition?

I'd find and trace them..

Re: kill switch wires

Yeah, see, that's the big question, because when I went to install the new kill switch, I figured there'd be some wires dangling somewhere from the old one, but I couldn't find any, so I looked in the headlight terminal and still couldn't find any. Also, are there supposed to be two kill switch wires coming from the HT coil, or just one? Because the wiring diagram I have shows only one, but there's a place for two (or three, third to ground), or something like that. Whatever, I must sleep now, but tomorrow I'll take off the HT coil/ignition coil (they're "integrated" see attachment #152; 153 and 154 are both lighting coils) tomorrow and see if I can trace the kill switch wires. I'll get back to you tomorrow, thanks.

Here's the attachment

Sorry, here's the diagram.


Re: another diagram

Normally, the HT coil, the points and the condenser are connected together. They may not be obviously, directly connected.. some wire's path might travel though an ignition switch first, for instance.. but for the engine to run the three things are connected.

If you ground the condenser wire the engine dies.. since the condenser wire is connected to the points, you're also grounding the points .. you're also grounding the HT's primary coil. It's the same difference.

Page 78 of THIS manual has a wiring diagram (or see attachment).

It shows two wires at the kill switch. The kill-wire is black. It's appears to be connected to the magneto's blue (points/condenser) wire at the HT coil..

This black wire appears to go through two "terminal blocks" on it's way up to the kill switch (called the "Cutoff switch" on a Batavus).

Funny thing.. Pages 76-77 of the manual instructs making a drawing before removing switches and the wires.. says it can be "very easy to forget how it was once they have been removed".


Re: another diagram

Yeah, this is the exact diagram that is in the book that the guy have me when he sold me the ped. See, it only has one wire kill wire. Anyway, I just spoke with my father, who is an electrician, and he said to start by replacing the points, which I ordered a while ago and have just been sitting in my garage, so I'll do that tonight.


Re: another diagram

i guess they grounded the ignition up at the handlebars themselves..

Dad's an electrician .. cool.. get one of his old meters! New points can't hurt. I'd still search for that cutoff switch wire and then tape and secure it's end.

Re: another diagram

Hello, sorry it's been a while since my last update. So I investigated much into this no spark problem. I replaced the new condensor this afternoon with another new condensor, and still no spark. I replaced the points tonight and got a big fat blue spark, so I figured it was just a matter of replacing the points. So after I tested the spark, I set the timing, though I set it _very_ inaccurately. I stuck the spark plug in and turned it over and it wouldn't start. After a couple times it make this high pitched squeek, like air being blown through a small hole, or a gasket perhaps. I took off the muffler and turned it over again. This time there was a huge back fire and an even bigger blue flame that shot out of the exhaust pipe. Seriously. Big huge blue flames shoot out of my exhaust pipe, and they're always accompanied by a backfire type of sound, sometimes louder, sometimes quieter. I have no idea. I'm thinking it may be incorrect ignitioin timing, like, way off ignition timing. Any Ideas? Thanks.

Re: Vacuum Leak

The back fire is due to a mis adjusted timing many people think adjust the breaker and you have adjusted the timing. This is wrong there the whole TDC thing or on a Puch the rottating of the base plate etc. Anyway I have a Peugeot the engine is very similar to Batavus engines as I have worked on both. You must get the timing right research and study till you fully understand the timing prosedure in any case do that and get it right. I have a saying to myself and friends "there is no Moped I can't get running well" so far this has been true. I have somthing to tell you that might solve your problem seeing as Peugeot and Bataves engines are similar, that is they are both reed valve engines. The spark thing I know you will eventualy get right, after that kick the engine over see if the plug is wet (it should be soaked in gas) that is assuming your plug does not fire. The idea is to find out if you are getting good gas flow to the engine, if not as I think is your problem. You have a air leak reed valve engines are more complex than piston ported engines and require a perfectly sealed crank case to work right. I was wondering if your engine runs verry hot (overheating) my bike has this problem and I have done everything to fix it. This is my theory engine overheats due to lack of oil this is caused by a air leak somewhere on the crankcase, this overheating causes condenser to behave erratically and or causes Carburator to overheat vaporizing petrol causing improper Carburator function. So look for oil from gas around your engine this will indicate a leak it must be fixed by replacing gasket or oilretaining ring or whatever is causing a leak. Hope fully you can see oil leaking if not the leak could be up higher and will be difficult to find. This overheating/airleak and condenser failure is just my thesis as to my Peugeot's problems, I thought it might apply to you. The engines are both Reed Valve engines and theroreticaly could be suffering the same problem. You may be suffering diffrent or added problems now as you have changed variables with playing around with the timing. In any case hope you can find the problem, locating and air leak is tricky and fixing it is a major undertaking requiring disassembly of the clutch or possibly the whole engine itself. GOOD LUCK

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