A rare (and vicious) Citroën 2CV carburator trouble

1. No more idle, the engine stops.
2. Very unstable engine at low speed, when the acelerator pedal is slightly down.
3. Perfect working engine at high speed, when the accelerator pedal is down enough.
4. and sometime (random !) engine is back to perfectly normal (idle OK, low speed OK), until all goes wrong again...

It looks like a dirty idle jet but if you check the jet, its clean and if you open the carburator there is nothing wrong.

So What ?!

We will first see how works the carburator in this area when there is no trouble, this will allow us to understand the trouble and fix it.

Here is a double barrel 2CV6 carburator :

On a single barrel the trouble studied here cannot happen, so if you plan to change a single barrel for a double one, don't. First, the accelerator command is not working on the same way at all (pushing, on the single barel; pulling on the double barrel, so you would have to make additional work on the pedal); Second, you would add a potential trouble to your 2CV, which is not such a good idea. And finally, the extra power coming from the double barrel is not so much and not worth the troubles of the modification.

How are working the two barrels ?

Basically a double barrel carburator is two carburator in one, you can clearly see this if you look at the carburator entries or exits : you see the two entries and exits (here, the first barrel, the smaller one, is on the left).

The first barrel is working up to some power then the second barrel is working to go over the power that can be delivered by the first one only. They are not working exactly that way in fact : the second barrel starts to act before the first one is at its max power.
When the engine is cold, the choke close the first barrel (the second is already closed as the first is not opened by the accelerator). Some air and gasoline goes to the engine through bypass holes.
Here is how it works on the 2CV carburator (a rigid cable pull to close and push to open)

(The studied trouble is not coming from the choke system)

When it works, it works that way :

At idle

The accelerator position. Note the spiral spring making a "hook"	The command of the second barrel (closed)	Both barrels are closed

With some acceleration

The accelerator position.	The command of the second barrel (closed) Note the "hook"	First barrel open Second barrel closed

With some more acceleration

The command of the second barrel starting to act	First barrel almost fully open Second barrel starting to open

At maximum acceleration

The command of the second barrel fully opening second barrel	First barrel fully open Second barrel fully open

The trouble

The spiral "hook" is used to close the second barrel, it is acting only when the accelerator is released. Now suppose that the spiral spring is broken, what will happen ? The "hook" will no longer act and so we will have this :

The explanations

1. No more idle, the engine stops because there is too much gasoline and air coming from the second barrel
2. Very unstable engine at low speed, when the accelerator pedal is slightly down because there is too much gasoline and air coming from the second barrel.
3. Perfect working engine at high speed, when the acelerator pedal is down enough because the second barrel should be open and IS open, so it is acting on its usual way.
4. and sometime (random !) engine is back to perfectly normal (idle OK, low speed OK), until all goes wrong again... because there is sometime enough friction and inertia on the parts (mainly if there is some dirt around) to move all the parts and close the second barrel when you all of sudden release accelerator.

Solving the trouble

If you can remove the carburator, its not too much difficult to fix the spiral spring with a strong and thin plier. But if you like mechanical 2CV challenge (guilty, your honor...) you can fix it with the carburator on the 2CV. I spent several hours on it... and finally used the other spring than the one which had the hook. In fact, that additional spring was not acting on anything (?)
The carburator used for the pictures also had that additional spring not acting on anything. What was the original work of that spring ? Mystery...

A trick

The following have nothing to do with the previous but as i had the carburator on its stand and the camera ready, i took the picture. Sometime, depending on the thickness of your spanner, you may have trouble to unscrew the idle jet. A solution is to cut of a part of the carburator, under the gasoline entry. Under the tube there is plenty of empty space, plus the metal of the carburator is quite soft and you can cut it easily with a short piece of metal saw blade.
The metal to cut-off is shown as yellow on the pictures below.

 

Date: Mon, 4 Oct 2010 17:39:58 -0400 Subject: Many Thanks! From: Jimmy Your tip about idling problems in the 2cv being related to dirty springs was immensely helpful. I drove for 3 long days (Taos New Mexico to Louisville, Kentucky) having to pump the gas until it got up to speed the whole way. On arriving, it refused to go further until fixed. I worked on it for two days until I saw your site and it was fixed straight away. Again, many thinks for your gratious sharing of your time and knowledge. Best wishes, Jim

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