Moby 7 runs for a while, then doesn't.

Talked a bit about this moped in another post but I'm gonna lay out everything I did here in one place --

I got a Moby 7 from a friend which ran after I cleaned out the carb (Sha 15.15) but was having some issues where sometimes it would work fine, sometimes it would only work with the choke on, and sometimes it would just die. I decided to clean the whole thing out after reading Fred's Guide, I took apart the engine, changed seals, bearings, gaskets, motor mounts, throttle cable, decomp cable and reassembled. It started up, but I was experiencing the same issue (needed the choke to run most of the time, except randomly it would work fine).

I thought "it's either not getting fuel or getting too much air". I sprayed starter fluid around anywhere that might leak and it didn't affect it, so I thought it's a fuel issue. Thinking "the jet looks clean but maybe it's clogged or the previous owner picked the wrong size, I stuck a 65 jet that I had just bought instead of the old 63 jet -- this seems to have fixed it! I rode it around maybe 1/2 a mile that night and 1/2 a mile one more other time but it's been raining all week so that's it.

Cue today: it's sunny and I want to go for a ride! Moped starts right up, everything seems good. After idling a while I turn it off to clean it up a bit (tighten loose throttle, shorten the new gas line that I made too long) and after that I try to start it and it won't start... I try to check if I have spark -- I can't see one (in the sun?) But my multimeter seems to be picking up a change on the plug once per rotation. I do notice that the plug is black and oily -- I clean the plug with a rag and stick it back in and it works! For 15 seconds, then it dies again, I pull the plug and it's oily.

Could my upjetting by 2 cause this? or bad oil/gas mix? Did I correctly troubleshoot my "choke" problem or did I just create a new problem instead. Maybe the spark is a red herring and something is changing when I leave it alone to cool down...

Starting to get frustrated, I wanna ride while it's 70 and sunny 🙄

20200502_161402.jpg

Re: Moby 7 runs for a while, then doesn't.

Forgot to mention-- everything besides the carb is stock. Also I'm a little confused about whether or not this metal screen that pops into the carb counts as an air filter, or if I should use the red one below it that I got online (or use both?)

20200502_163043.jpg

Re: Moby 7 runs for a while, then doesn't.

If it stops running after idling for a while and the plug is wet, that’s a fouled plug.

If everything is stock, it should have the stock jet. Too rich and it might let you ride it at anything but idle. But, drown that plug at idle.

No leaks, wet plug, down jet.

Sounds like it was upjetted before to compensate for a leak and now it’s too much fuel after the rebuild.

Re: Moby 7 runs for a while, then doesn't.

Plug is bad as stated. You could also double check your ground wire for the engine. It needs to have a solid ground to the frame which is accomplished by a wire not the engine mounts cuz of the rubber bushings. If that wire is iffy french bikes die.

Re: Moby 7 runs for a while, then doesn't.

Just a few observations because some big problems are a combination of a bunch of little shytes.

SHA 15:15 on a stock AV7 needs to be jetted with around 59-62 depending on which factory cylinder you have, so your jet is very fat.

The stock intake on a Ranger 7 is 13,8mm bore, so it is under size for the 15mm carb You will likely want to ream out the intake to match the bore of the carb or have a terrible time with jetting.

Pick your screens - stock inserted of the fancy red one. If you go for the red one,carry the other cartridge as a spare on the bike in case the other one decides to go AWOL.

You will likely want to clean out the exhause,the stock peashooter exit pipe on the backing plate must be pretty gummed up after running cold and wet for a while. You might want to consider derestricting the exit pipe as well.

The spark should be visible in just the shadow of the bike as a nice bright blue snappy spark. I fit is yellowy and anemic you would want to change out the condenser and do the external condenser mod. That poor component cooks inside the Novi unvented magneto and heat is the enemy of capacitors.

Also as Jay suggested, replace the bonding strap so the frame and engine has good electrical continuity.

Re: Moby 7 runs for a while, then doesn't.

Thank you guys for all the advice!

Before the comments -- I had put in a brand new plug and it started right up.

>> If it stops running after idling for a while and the plug is wet, that’s a fouled plug.

Is there any recommended way to clean a fouled plug? I tried wiping it down with carb cleaner and it started for a few seconds, but the new plug ran for a while. It definitely seems to be fouling up after idling, and after playing with the new plug it seems to work for a while, then it dies after idling for a while.

>> Sounds like it was upjetted before to compensate for a leak and now it’s too much fuel after the rebuild.

That could very well be the case. When I got the moped there were a lot of potentially leaky areas -- I'll order some smaller jets.

>> You could also double check your ground wire for the engine.

When rebuilding I bolted the ground wire to the engine mount, and it seemed like a fairly good connection. There's also another ground wire going from the harness to the frame, underneath the rear wheel well, which I removed and bolted to the top because it was in the way of the 16x2.25" tire I installed. I'll take a look at the quality of the whole wire but I don't think there's a bad connection there.

>> SHA 15:15 on a stock AV7 needs to be jetted with around 59-62 depending on which factory cylinder you have

How would I tell what cylinder I have and what jet size that corresponds to?

>> The stock intake on a Ranger 7 is 13,8mm bore, so it is under size for the 15mm carb You will likely want to ream out the intake to match the bore of the carb or have a terrible time with jetting.

Is this the intake on the head, or the pipe that connects the carb to the head? I'll take a look at this once I get it running consistently.

>> Pick your screens - stock inserted of the fancy red one.

Red one doesn't fit well under the side covers and falls off easily so I'll go with stock if it doesn't matter that much. I don't have the stock cover.

>> You will likely want to clean out the exhause,the stock peashooter exit pipe on the backing plate must be pretty gummed up after running cold and wet for a while.

I took apart the exhaust, but didn't spend too much time on it because I was hoping to attach a Ninja G3 pipe I bought, but then I realized that the Ninja Pipe is almost touching the ground on the Model 7 (apparantly it's designed for a mounting angle more like a 50V... Sad cause I spent a lot on it!) I didn't "throw it in the fire pit" but I sprayed it down with some degreaser and cleaned it out -- The exit pipe didn't seem to be really gunked, but checking again may be worth it.

>> You might want to consider derestricting the exit pipe as well.

As above, I was going to upgrade the pipe before I realized the upgraded pipe wouldn't work with this moped -- Now that I can't do that I'll look more into derestricting this one.

Spark was blue last time I checked -- It was reaaaly bright out today and I was checking that dirty spark plug, so I might not have seen it well.

Re: Moby 7 runs for a while, then doesn't.

No there is no way to De-foul a plug. It is bad at an internal level when they go out. It will need to be replaced. I typically just buy out my local auto parts store of they b7hs so that I always have plenty of spares. My kitted bikes I always carry an extra plug because it is no fun fouling a plug during your ride. And kitted bikes foul plugs a lot faster than stock bikes. If you find you are fouling plugs often move a step up. B8hs or even a b9hs those handle higher temps. Can be a little harder to find tho.

The engine mount is not the place to ground your engine. As I mentioned the French bikes have rubber bushing that the engine mount bolts go through. This will not ensure an adequate ground. Move the wire up to the next spot on your frame you can put a good bolt on. Even chip of a touch of paint where you are bolting it to ensure you are connecting with the metal really well. Just a suggestion because it will be your gremlin even if it isnt at this moment.

Re: Moby 7 runs for a while, then doesn't.

Jay -- The wire connects to the engine itself at the engine mount bolt (why does the rubber matter as long as the strap has a solid connection to the engine's mount bolt itself? and the other end of the wire is connected elsewhere on the wiring harness (bolted to the frame with some other cables IIRC)

Re: Moby 7 runs for a while, then doesn't.

I also noticed that the idle screw position that lets it "run without dying" changes with the temperature of the engine. Having it screwed in all the way is the only position that consistently idles, but even then sometimes the engine dies with the brakes on or wheels on the ground. It seems to be "better" when the engine is warmed up but not always. Is this probably just related to running too rich, and changes in temperature making it slightly more rich and starved for air?

Re: Moby 7 runs for a while, then doesn't.

>>Before the comments -- I had put in a brand new plug and it started right up.

If the plug still fouls at idle after fixing all the smaller troubles then get a hotter plug so it burns off the deposits while running.

>>How would I tell what cylinder I have and what jet size that corresponds to?

You need to open the engine and measure the transfer size. Instead it is so much easier to have jets in the 59-62 range and see which one gives you a goodly colour on the spark plug.

>>Is this the intake on the head, or the pipe that connects the carb to the head? I'll take a look at this once I get it running consistently.

The intake pipe between the carb and the cylinder is where you need to open the bore. Because of the difference in the bore sizes 15mm va 13,8mm you will have a terrible time getting it to run consistently.

>>Ninja G3 pipe I bought, but then I realized that the Ninja Pipe is almost touching the ground on the Model 7 (apparantly it's designed for a mounting angle more like a 50V... Sad cause I spent a lot on it!)

That is a very nice pipe - instead of a heart break you could take it to a welder and have then section and reweld part of the header to adjust the angle so it fits tight to the undercarriage of the bike.

Stock exhause is only 8-9mm inside diameter, it is a restriction to keep noise and speed to regulatory compliance. It is designed to work with 12mm Gurtner carb and not a 15mm carb. Replacing the exit with a 11m ID pipe would improve the breathing of the engine.

Re: Moby 7 runs for a while, then doesn't.

Thanks! Lot's of little things for me to keep working on. Gonna start with jetting and make sure it at least works consistently and then derestricting the intake/outtake. Though -- Once I bore out the intake and exhaust I'll probably need to upjet back to 63-65ish if I understand correctly?

Re: Moby 7 runs for a while, then doesn't.

Yes, 59-62 range is for the right size intake and stock exhause. Also my suggestions are always a little on the rich end of the scale, so your "cafe au lait" is not too much milk in it, but still brown not black cafe for a spark plug chop.

> Alex Gribov Wrote:

> -------------------------------------------------------

> Thanks! Lot's of little things for me to keep working on. Gonna start

> with jetting and make sure it at least works consistently and then

> derestricting the intake/outtake. Though -- Once I bore out the intake

> and exhaust I'll probably need to upjet back to 63-65ish if I understand

> correctly?

Re: Moby 7 runs for a while, then doesn't.

Hey Rebel, had a couple follow-up questions. I got the Ranger 7 running pretty good stock, but then recently I've had a couple complications. I was running a B7HS plug with a 61 Jet and it was running well, and recently it started having trouble idling when cold. I compared the spark on the plug to a new B6HS I had laying around and noticed it wasn't sparking at low RPMs while the new plug was (does that mean it's... partially fouled? ) Anyways, I decided to finally run a plug-chop -- Warmed up for 4 miles, put in a new plug, ran about 0.75 miles on WOT and killed the engine -- The plug still looks brand new. What's up with that? Does it have to do with the fact that I'm killing the engine using the decomp instead of the killswitch (I don't have a working kill switch right now).

Also you mentioned in another post that rather than getting a 70cc kit, which would be too powerful for this bike I could just port my existing 50cc. Would ordering an airsal 50cc kit achieve a similar thing as porting mine? What improvements would one or the other give me (extra hill climbing power, maybe an extra 5mph?) I'm going to try to weld the header at the new angle for my Ninja exhaust this week, so I'm starting to think about giving the engine a little more oomph after that.

Re: Moby 7 runs for a while, then doesn't.

as far as idling, poke a small wire thru the idle jet, its below your sha carburetor slide, blow it out.

having a brand new looking plug at wot is no good. post a picture, should look brown.

air leak search.

Re: Moby 7 runs for a while, then doesn't.

Yes you could get an arisal 50cc kit but porting yourself is sooo easy with a couple easy to find tools. Slapping a kit on is not always easier. After I ported my first engine I felt a lot more confident in working on my bike. I felt like I knew it better.

One more thing to check if it hasnt been mentioned is your condenser. I love replacing my condenser with an inline one off of the stator plate. A karmenghia condenser from autozone is perfect for this.

Re: Moby 7 runs for a while, then doesn't.

> Jay Rivett Wrote:

> -------------------------------------------------------

> A karmenghia condenser from autozone is perfect for this.

Bosch brand is better than any no name .

I've never had the best luck with Autozone electric anything .

But , maybe they increased their quality over the years .

I don't know , as I quit buying anything electric from them a long time ago and now buy known excellent quality brands . ;)

Re: Moby 7 runs for a while, then doesn't.

After running a brand new plug hard for 3/4 mile and killing the engine that plug's insulator, at the electrode, should still look brand new. If it doesn't that plug is too "cold". Please read this until you understand it. Especially this part, "So how can you tell whether you've chosen the right heat range? It's easy: a spark plug should be getting hot enough to keep its insulator nose completely clean, with all deposits burned away, but not so hot that its electrodes show signs of serious overheating. These are things to look for on a new plug that has been subjected to a few minutes of hard running. After many miles of service insulators acquire a coating of fuel deposits, with some coloration from oil in two-stroke applications, and there will be some erosion of the electrodes even when everything is normal. Don't try to read old spark plugs; even the experts find that difficult. New plugs present unmuddled information about what's happening inside an engine, and can give you a complete picture after just minutes of hard running. At least they will if they're running hot enough, and that should be hot enough to keep the insulator clean". http://www.strappe.com/plugs.html

The picture below shows where to look to "read" a plug.

plug_section_16.gif

(edited)

Re: Moby 7 runs for a while, then doesn't.

Thanks guys -- A few responses and pictures of plug in question:

>> poke a small wire thru the idle jet, its below your sha carburetor slide, blow it out.

-- I'll give this a shot. I also ordered a le partie CDI because I've read a lot about Motobecane points wearing out -- I seem to be skipping sparks once in a while by the sound.

>> Yes you could get an arisal 50cc kit but porting yourself is sooo easy with a couple easy to find tools.

Still thinking about this -- I didn't order a it yet and I'll consider porting. Honestly I just dreaded having to set the timing again but I think it'll be easier now that I got a CDI and a proper micrometer coming in the mail. It's funny, I hear some say what you did, and some say "don't try your first port unless you've got a spare engine"

>> One more thing to check if it hasnt been mentioned is your condenser.

THIS is actually another question I had -- I just bought one thinking mine might be worn. After reading some posts here I ordered this one . Sanded off the paint on my frame under the gas tank (there was already a hole there) and bolted it on and wired in place of my kill switch connector -- When I have it connected the engine dies unless I'm at >50% throttle. I read that it's okay for it to be in parallel with the old condensor, but is it possible that if my old condensor is good I've now got too much capacitance?

I added a picture of the spark plug, though since I made that last post I modified my Ninja G3 pipe to mount (pic also attached) so I should probably upjet and run another plug chop. Based on the plug picture (from before the pipe) would you say my previous jetting was off? So far my low end is much better with the pipe (so much that my belt is noticeably slipping now! Time to get a new one).

20200519_234655.jpg
20200520_150643.jpg

Re: Moby 7 runs for a while, then doesn't.

Also I just noticed -- I had orderd BH6S plugs from treats but the plugs I got all say BH9S...

Re: Moby 7 runs for a while, then doesn't.

> Alex Gribov Wrote:

> -------------------------------------------------------

> Also I just noticed -- I had orderd BH6S plugs from treats but the plugs

> I got all say BH9S...

Don't use them. They don't get hot enough to keep clean. The BP6HS plugs are available at the auto parts store up the street for probably less than half the price of mail order.

Re: Moby 7 runs for a while, then doesn't.

> P D Wrote:

> -------------------------------------------------------

> > Jay Rivett Wrote:

>

> > -------------------------------------------------------

>

> > A karmenghia condenser from autozone is perfect for this.

>

> Bosch brand is better than any no name .

>

> I've never had the best luck with Autozone electric anything .

>

> But , maybe they increased their quality over the years .

>

> I don't know , as I quit buying anything electric from them a long time

> ago and now buy known excellent quality brands . ;)

No doubt Bosch is the way to go, however I only mentioned aoutozone cuz that was where we were able to find the condenser on the fly. You know when your bike doesnt start an hour before the rally ride. ;j

Re: Moby 7 runs for a while, then doesn't.

> Jay Rivett Wrote:

> -------------------------------------------------------

> No doubt Bosch is the way to go, however I only mentioned aoutozone cuz

> that was where we were able to find the condenser on the fly. You know

> when your bike doesnt start an hour before the rally ride. ;j

Agreed . Sometimes you have to take what you can get .

Re: Moby 7 runs for a while, then doesn't.

If you put a external condenser you must remove or at least cut the wire for the old one. Wire is the Blue with Black stripe going to the points.

> Alex Gribov Wrote:

> -------------------------------------------------------

> Thanks guys -- A few responses and pictures of plug in question:

>

> >> poke a small wire thru the idle jet, its below your sha carburetor

> slide, blow it out.

>

> -- I'll give this a shot. I also ordered a le partie CDI because I've

> e read a lot about Motobecane points wearing out -- I seem to be

> skipping sparks once in a while by the sound.

>

> >> Yes you could get an arisal 50cc kit but porting yourself is sooo

> easy with a couple easy to find tools.

>

> Still thinking about this -- I didn't order a it yet and I'll consider

> porting. Honestly I just dreaded having to set the timing again but I

> think it'll be easier now that I got a CDI and a proper micrometer

> coming in the mail. It's funny, I hear some say what you did, and some

> say "don't try your first port unless you've got a spare engine"

>

> >> One more thing to check if it hasnt been mentioned is your condenser.

>

> THIS is actually another question I had -- I just bought one thinking

> mine might be worn. After reading some posts here I ordered this one.

> Sanded off the paint on my frame under the gas tank (there was already a

> hole there) and bolted it on and wired in place of my kill switch

> connector -- When I have it connected the engine dies unless I'm at >50%

> throttle. I read that it's okay for it to be in parallel with the old

> condensor, but is it possible that if my old condensor is good I've now

> got too much capacitance?

>

> I added a picture of the spark plug, though since I made that last post

> I modified my Ninja G3 pipe to mount (pic also attached) so I should

> probably upjet and run another plug chop. Based on the plug picture

> (from before the pipe) would you say my previous jetting was off? So far

> my low end is much better with the pipe (so much that my belt is

> noticeably slipping now! Time to get a new one).

Re: Moby 7 runs for a while, then doesn't.

> Rebel Moby Wrote:

> -------------------------------------------------------

> If you put a external condenser you must remove or at least cut the wire

> for the old one. Wire is the Blue with Black stripe going to the

> points.

>

That^ is logical : why would you leave a non working part in place to leech from

or impede the new part ?

Re: Moby 7 runs for a while, then doesn't.

> P D Wrote:

> -------------------------------------------------------

> > Rebel Moby Wrote:

>

> > -------------------------------------------------------

>

> > If you put a external condenser you must remove or at least cut the

> wire

>

> > for the old one. Wire is the Blue with Black stripe going to the

>

> > points.

>

> >

>

> That^ is logical : why would you leave a non working part in place to

> leech from

>

> or impede the new part ?

Best to not assume anything with moped repair. But yes you can just snip the wire that leads to your condenser and run an extension from that to the new condenser and then to your coil.

It will be the wire coming from your points.

Re: Moby 7 runs for a while, then doesn't.

Get B6HS not BP6HS - the projected nose on the BP6HS will not help out

> Den 23 Wrote:

>

> Don't use them. They don't get hot enough to keep clean. The BP6HS plugs

> are available at the auto parts store up the street for probably less

> than half the price of mail order.

Re: Moby 7 runs for a while, then doesn't.

For your porting/kitting questions:

Renewing the spi seals is important for any mods on the engine for higher output. Also a goodly measure of patience will help you get the best out of your choice, read informations available and understand them first. Ask questions to folks who have done it before successfully and follow their lead.

Stock AV7

A well kitted stock ported AV7 will give you higher revs andabit better power band. Someone has one I suggested posted on the wiki somewhere, that port event recipe will get someone in the 30-40mph range if they have a goodly pipe, SHA15:15 with 15mm intake and matched intake duct, 1,5mm BTDC timing (1,2mm for CDI because hotter kernel). It will give you reliability all day long and into the night.

There are other port event recipes for a av7 that will give you higher revs, but you will start to move the power band too high and would lose idle ability and pushing up higher starts to risk the reliability day to day.

Aftermarket AV7 50cc

Better port timing events rightout of the box for you. Also the casting has allowed for more robust castings which allows for more space to add better bigger ports.

Aftermarket AV7 overbored kits

Same as the 50cc kits, better port timing events rightout of the box for you. Also the casting has allowed for more robust castings which allows for more space to add better bigger ports.

Added bonus of more piston crown surface area so you have added torque, that will yield you a more robust power band and less scrubbing off of power on hills.

NOTE: all and any aftermarket kits will need you to index the cylinder to the engine, you would also need to go over the finish of the kit and give it some quality work, deburring, chamfer on the port windows, attending to casting marks in ports as well as polishing the exhause port. Matching the cylinder to the carter to remove any shelfs and ledges that can cause fuel stripping and puddling. Better port timing events rightout of the box for you. Also the casting has allowed for more robust castings which allows for more space to add better bigger ports.

You will also need to consider a good exhause as well as a multi circuit carb bigger than 15mm.

Re: Moby 7 runs for a while, then doesn't.

Michael Branscombe /

when it quits, pull the plug and see if the spark is still snappy and blue. I did an old Rotax a few year ago - and chased all sorts of problems. I wasted a lot of time on things it wasn't, it just needed a new secondary coil. symptom was fouling a plug after 1/2 hour or so. maybe 10 minutes, depended on the day and the weather. Spark would jump 1/3 inch gap, but the spark on the running plug was not snappy and blue, it was orange. New coils, never fouled a plug since. Tested fine when cold, but heat things up and all bets are off.

Re: Moby 7 runs for a while, then doesn't.

> Rebel Moby Wrote:

> -------------------------------------------------------

> Get B6HS not BP6HS - the projected nose

> on the BP6HS will not help out.

I'd like to read the reason that the BP6HS plug will not help out. Please post a link to a reference.

Re: Moby 7 runs for a while, then doesn't.

Pick your own reference for flame kernel propagation if you like to get technical.

The head is more efficient at cooling than the cylinder. If the kernel of the flame is closer to the piston crown the heat generated will soak more into the crown than the head. The only method to cool the crown is by transferring the heat to the cylinder cooling fins (or water jacket if it is a jacketed cylinder) via the piston ring or rings. Excess heat via the rings can start to boil off the oil between ring and cylinder, no oil will allow the ring to micro weld to the cylinder wall, providing seize, scar or shadow results. Without the projected tip the heat of the combustion is located closer to the head where it is a more direct transfer to the head fils or cooling jacket in a liquid cooled engine.

> Den 23 Wrote:

> -------------------------------------------------------

> > Rebel Moby Wrote:

>

> > -------------------------------------------------------

>

> > Get B6HS not BP6HS - the projected nose

>

> > on the BP6HS will not help out.

>

> I'd like to read the reason that the BP6HS plug will not help out.

> Please post a link to a reference.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account