Mobylette Gurtner Carb

Dee Brann /

Jul 08, 2020 08:10AM

A couple of months ago I bought a 76 Mobylette. I’ve been slowly restoring it mechanically to get it back on the street. It’s been going pretty well except for a couple of issues.

I can’t seem to get a consistent strong spark. I replaced the points and condenser, set the gap and all looks good. After double checking everything, I changed the coil with one I had laying around, got good spark and started the bike. It would run with the choke on, but not at all with the choke off.

I discovered the main fuel passage that goes from the lower part of the fuel bowl to the main jet and it was plugged with trash.

Now, here is what I don’t understand. How could the Gurtner design ever work? I must be missing something, because I don’t know how fuel can go from the bottom of the bowl directly to a main jet that is open on the bottom. As soon as fuel starts filling the bowl, it starts running out the bottom of the main jet. I’m sure I’m missing something that is installed wrong or ??? Thanks for the help in advance.

Re: Mobylette Gurtner Carb

Mike McScoutington /

Go to branch

miker

Jul 08, 2020 01:44PM

Gurtners are trash. Do yourself a favor and throw a Dellorto SHA 15:15 on there.

Re: Mobylette Gurtner Carb

Pushrod Fifty /

supercub

Jul 08, 2020 02:10PM

The thing on the bottom that looks like a jet is just a plug. Under that is a threaded ring with a shallow screwdriver slot that can strip out real easy. The ring holds the jet in and the emulsion tube. Best to give the innards some pb blaster and let soak for a day before trying to unscrew the ring.

Re: Mobylette Gurtner Carb

Dee Brann /

raisin75 OP

Jul 08, 2020 02:58PM

Pushrod Fifty, I understand there is the emulsion tube which protrudes into the carb throat, which is held by the threaded ring. Then, at the very bottom is the main jet. Unless mine is wrong, my main jet is open on the bottom with a fairly large hole. I’ll get a pic this evening.

Re: Mobylette Gurtner Carb

Dee Brann /

raisin75 OP

Jul 08, 2020 08:54PM

Here’s a couple of pics of the main jet. No plug, just a hole straight out the bottom. From what I understand this is right, but tell me if I’m missing something.

Re: Mobylette Gurtner Carb

cheetahchrome . /

cheetahchrome

Jul 09, 2020 07:39PM

No that jet is whacked. It should have a steel ball that is forced into it after the machine work is done. Yours is missing Hmmm. Unless the steel ball is just forced down in there further than most. If the steel ball is in there all is Zen.

Most of mine look like this

BTW I absolutely love Gurtner carbs and use them exclusively on all my 103 builds. Once you learn all the tips and tricks to make the idle circuit work they are faultless and can make a stocker go 37.

You just have to read and learn and not give up and soon all will be revealed and clear. (edited)

Re: Mobylette Gurtner Carb

Dee Brann /

raisin75 OP

Jul 09, 2020 08:42PM

That’s it! Finally, I can see what is missing. Thank you so much for the help. I’ll either get another jet or find a steel ball to put in this one. Now I can get on with my life and finish this project.

Re: Mobylette Gurtner Carb

P D /

pdcomm

Jul 09, 2020 09:15PM

> Dee Brann Wrote:

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

> That’s it! Finally, I can see what is missing. Thank you so much for the

> help. I’ll either get another jet or find a steel ball to put in this

> one. Now I can get on with my life and finish this project.

Consider solder in place of the missing ball .

Re: Mobylette Gurtner Carb

Pushrod Fifty /

supercub

Jul 10, 2020 05:11PM

Ok so thats the actual jet. The ball on this one is way inside.

Re: Mobylette Gurtner Carb

Dee Brann /

raisin75 OP

Jul 10, 2020 05:55PM

I found a broken brass idle air screw from one of my Honda motorcycles that was just a bit bigger than this hole. I chucked it in my drill and formed a taper on the end of it, cut off a short piece and drove it into the main jet with a punch. It sits flush with the inner slot for the screwdriver and seems to seal well. Once I verify that my idle circuit is clean, I will put everything back together, turn on the petcock and do a leak test.

Thanks again for all the help!

Re: Mobylette Gurtner Carb

cheetahchrome . /

cheetahchrome

Jul 10, 2020 10:12PM

Just buy a new jet with the same number they are relatively cheap.

Treatland Gayboy jets

Re: Mobylette Gurtner Carb

P D /

pdcomm

Jul 11, 2020 07:33AM

> cheetahchrome . Wrote:

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

> Just buy a new jet with the same number they are relatively cheap.

>

> Treatland Gayboy jets

But , why , if he can make the old one work ... ;)

Re: Mobylette Gurtner Carb

Dee Brann /

raisin75 OP

Jul 11, 2020 07:47AM

It works great! I started it up yesterday afternoon. It needs some adjustment on timing but it’s running and revving. Doesn’t want to idle either, so I’m not sure if I got both idle passages clear. The one over the screw is easy, but the other is a bit hard to check.

I am going to have to replace the coil, too. The original gives a weak spark, and the one I’m testing with isn’t the right fit.

MOPED ARMY

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