Moby just won't run

mike thompson /

I picked up a 77 moby 50v. It came with an old cdi and what looks like a new SHA 15 carb with a 66 jet. The cdi uses the original cream colored Novi HT coil. I cleaned out the carb and put in new gas. It gets a ton of blue spark and the spark happens what appears to be about 10 degrees before TDC. The exhaust is not clogged. The compression holds at 90 psi all day. The plug is a little wet when I remove it after spinning it over with a drill. Gas drips out where the exhaust is screwed in when I am turning it over. It won't even sputter, nothing. Plug comes out is is a little wet but not brown or anything - it still looks like new. Nothing happens when I put gas or starter fluid directly into the cylinder. Does anyone have any thoughts?

Re: Moby just won't run

Try a known good plug .

Some plugs simply won't fire when under pressure/compression . They are either easily fouled or the spark gets 'blown out' before it can ignite any mixture .

Re: Moby just won't run

mike thompson /

I tried two different brand new plugs

Re: Moby just won't run

I'm not clear on whether you set the timing or not. How's the air filter? Remove the filter and try. Remove the entire exhaust and try.Edited 1 time(s). Last edit at 12/24/18 09:50PM by den23.

Re: Moby just won't run

sounds flooded, any evidence of combustion? pops, trys to start, black plug, etc. try cleaning plug, turning off petcock, wide open throttle. and drill spin for a couple mins to clear out. clean all connections, especially grounds.its always the damn crank seals.

Re: Moby just won't run

mike thompson /

I didn't set the timing, I checked it. I used a timing light and marked both the flywheel and the case where it fired and then put a rod in the spark plug hole and eyeballed that it was a little before tdc when the lines matched up.

The air filter is just wire mesh and the black plastic cover.

Re: Moby just won't run

mike thompson /

No evidence of combustion. No coloration on the plug. Doesn't even try. I have removed the plug each time and dried it off.

Re: Moby just won't run

Classic bad crank seals,maybe. Replace them anyway.

Re: Moby just won't run

mike thompson /

If it had bad crank seals, wouldn't it still run when I put starter fluid or gas directly into the cylinder?

Re: Moby just won't run

> mike thompson Wrote:

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

...a rod in the spark plug hole and eyeballed...the spark happens what appears to be about 10 degrees BTDC.

=================================================

I think I found a repair manual for your bike.

https://vintagemopeds.files.wordpress.com/2013/01/motobecane-mobylette-repair-manual.pdf

The manual indicates that the timing should be 1.5mm BTDC.

ScreenShot067.jpg

Eyeballing probably isn't accurate enough. I didn't know exactly how many degrees equate to how many millimeters in your engine, but it seemed to me that 10 degrees is not enough advance. If you know the stroke and the con-rod length of your engine you can use this calculator to determine exactly where you are.

https://www.scooterhelp.com/tips/timing/timing.calc.html

I played around with the calculator using different figures and I found that somewhere around 17-18 degrees BTDC should give you somewhere around 1.5mm BTDC.

If you have the proper gauge to fit into the spark plug hole you can correctly re-time the ignition to 1.5mm BTDC. Or, if you only have a degree wheel bolted to the end of the crank, you could try re-timing the ignition to 17-18 deg. BTDC. This is not exact but it should equate to somewhere near 1.5mm BTDC.Edited 1 time(s). Last edit at 12/29/18 11:51AM by den23.

Re: Moby just won't run

I just found another manual on this site...

https://www.mopedarmy.com/w/images/6/61/Motobecane_manual.pdf

...and it indicated that the timing should be set at 2mm BTDC.

ScreenShot068.jpg

Using the 2mm figure would equate to about 20deg. BTDC.

Re: Moby just won't run

mike thompson /

Hey, that was really nice of you. Putting all that work into that response. I appreciate it. I just got a micrometer Style timing tool from treats and I'm going to try it out today.

Re: Moby just won't run

mike thompson /

I replaced HT coil with a new generic one (spark looks amazing). I used a timing micrometer to time it to 2mm BTDC (it has a CDI ignition). I removed carb filter and exhaust (I blew through the exhaust pipe and there is no blockage), I cleaned out the fairly new looking SHA 15.15 carb, I used a known good plug (plug gets wet plus I tried starter fluid in spark plug hole). Cylinder has 150 psi compression. I can't even get it to sputter. When the exhaust was off, it looked like there was exhaust smoke coming from the exhaust port. Gas drips out the exhaust port. I am getting discouraged. I have not replaced the crank seals or the cdi box or the cdi ignition itself but the spark does look very strong and like purple.

If the plug is getting wet (or me using starter fluid), doesn't that rule out the seals?

Re: Moby just won't run

Sounds like it's getting too much fuel .

Re: Moby just won't run

mike thompson /

I know the guy who has it before me. He was riding all over with this carb. He was out on it and it was fine, then it started running crumbly for a while, then it quit. So he wasn't getting too much gas.

Re: Moby just won't run

> P D Wrote:

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

> Sounds like it's getting too much fuel .

^ This. Yes,it should fire even if blown crank seals if you use starting fluid. You must be drowning the plug.

Re: Moby just won't run

mike thompson /

I got it to run. I set the timing 2.5mm BTDC to do it. It ran for about 10 minutes and fouled the plug. It makes me feel like it is getting too much fuel. Not sure how to give it less fuel.

Re: Moby just won't run

Make sure the float height is right, they leak if not, stock gurtners anyway.

Re: Moby just won't run

Are you sure someone didn't drill the jets larger ?

Re: Moby just won't run

mike thompson /

The jet is a 66 which does seem big (although my puch bing has a 78). Does the jet size matter on start up? Doesn't it just matter at WOT?

Re: Moby just won't run

> mike thompson Wrote:

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

> The jet is a 66 which does seem big (although my puch bing has a 78).

> Does the jet size matter on start up? Doesn't it just matter at WOT?

I'm not the expert I'd like to be , but ...

Given that it's a new carb , let's assume all jets are as stated and as they should be .

FYI : the idle jet is feed via the main jet .

So , make sure the slide is all the way down ( not obstructed or held up and there should be a tiny bit of play in the cable ) and that the needle valve is copacetic .

By that , I mean the needle should seal completely when closed .

You might try polishing the needle seat with a q tip and Brasso or old style tooth paste . You want the seat to be shiny smooth with no imperfections .

The needle tip should be supple and smooth with no irregularities .

You should read the wiki and read the 'out of control' blog page :

https://www.mopedarmy.com/wiki/Dellorto_SHAEdited 1 time(s). Last edit at 12/30/18 12:33PM by pdcomm.

Re: Moby just won't run

mike thompson /

I can't get it running again. I sputters or pops once in a while. Tried two carbs, multiple plugs and timing settings. Now it looks like there are bubbles and fuel coming out of the head gasket in front. Could I have done that last night because the timing was so advanced? I get a 120 psi compression reading - is that possible with a head gasket that leaks gas and air?

Re: Moby just won't run

How did you gain 30# compression ?

And , why haven't you cured the head gasket leak ?

Re: Moby just won't run

mike thompson /

I believe I gained the compression because I got a missing clip that holds the decomp valve closed tight whereas it was missing before.

I just noticed the head gasket leak. I don't know if it is new. I am still getting the high compression reading so that is why I asked if it is possible to get a high compression reading with a head gasket leak.

On a Moby, the entire motor has to be removed to remove the head.

Re: Moby just won't run

I'd think it should run on 120# compression .

But , an air leak could every easily cause seizure .

I wouldn't even try running it with a leak . Just buying for trouble .

Re: Moby just won't run

mike thompson /

I took off the motor. There was no head gasket. Head mounting bolts were very loose. I see puffs of smoke coming out where a head gasket should be when I turn it over. My question is is a scorched piston a big deal? See attached.

20190101_175422.jpg

Re: Moby just won't run

What I see in the picture looks normal. I would take the piston off and remove the rings and clean the groves and the piston. I would use Scotch Brite and brake cleaner.

When you say "Head mounting bolts were very loose" do you mean the nuts or the studs? The nuts, of course, should be correctly torqued. The studs should be tight enough so they don't unscrew when you're trying to remove the nuts.

It may be normal not to have a head gasket; check your manual.

Re: Moby just won't run

mike thompson /

Sorry, I meant nuts. The bolts were fine. I will check the manual about the head gasket

Re: Moby just won't run

I don't see a base gasket or signs of there being one .

The case is dirty enough to make me think there was a big leak at the base of the cylinder , as well .

Re: Moby just won't run

mike thompson /

No, there is a nice fat blue base gasket. It is attached to the bottom of the cylinder. Base surface is just dirty from handling. I have a new base gasket on order anyway.

Also (getting back to an earlier question), the manual shows that there should be a head gasket.

Is it possible to get a high compression reading and still not run because of a leak where the head gasket should be? It doesn't seem possible. Also, I would say that if the base gasket was leaking, I would not be getting a wet plug.

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