Automatic Choke Pull-Down Fault - BX 19TRS Solex 34-34 Z1

[ellevie]

Automatic Choke Pull-Down Fault - Citroen BX 19TRS Solex carburettor 34-34 Z1

Problem:
On cold start, instead of fast idling the engine revs are so low that the accelerator has to be used to keep the engine from cutting out. Black exhaust smoke instead of white smoke.

Cause of problem:
Vacuum leak between choke pull down diaphragm housing and reservoir in choke cover.

Solution:
Wrap a few turns of PTFE tape around the spigot which connects the reservoir to the diaphragm housing.

Details:
At cold start the choke flap at the top of the left hand venturi chamber is fully closed to give a very rich mixture and must be opened about a quarter of an inch within about one second of the engine firing to allow some air through, otherwise the engine suffocates and the mixture is too rich preventing fast idle and causing black smoke. The carburettor air intake cover can be loosened and pulled aside to check if the choke flap is not opening just after firing. If it fails to open it can be pressed down slightly using the finger after engine firing to confirm that fast idle is restored. The pull-down mechanism operates as follows. Observe a thin vertical 3cm spring in the left of the housing the top of which is connected to a lug that pulls the choke flap closed when cold. The pull-down diaphragm on the left of the housing is actuated by manifold vacuum to lift the lug upwards via a brass rod and lever. The black choke plastic cover contains a reservoir which is connected to a hole in the side of the diaphragm housing via a short spigot projecting from the cover. Air must first be sucked out of this reservoir before sufficient vacuum can be achieved to move the diaphragm thus providing a short delay of the order of a second or so before the choke is pulled down. The correct operation of this mechanism can be observed with the choke cover removed by placing your finger over the hole in the diaphragm housing to form an airtight seal when the lever will clearly be seen lifting the lug. This can only be seen when cold as the thermostatic capsule lifts this lug to the full open position when the engine is fully warmed up. Faulty operation can be checked by turning the cover containing the reservoir through 180 degrees and inserting the lug in the hole. This spigot looks as if it originally might have had a small o-ring to form a seal but a few turns of PTFE tape works well.

The Haynes manual suggests that there are various versions of the diaphragm arrangement with a number of tubes connecting to it, leading to even more possibilities for vacuum leaks which would have to be investigated. Perhaps the simplest thing to do would be to disconnect the tubes and block the holes in the diaphragm housing at the expense of removing the intended delay. For example, discarding the reservoir and blocking the diaphragm housing hole in my choke resulted in the engine firing but immediately cutting out, and then firing ok on the second attempt. A quick dab of the accelerator on the first attempt kept it going, a small price to pay.

[jeremy]

Sounds like a good afternoon's work. Bit late I know but I think you'll find the official Citroen manual for your carb here

http://www.rwbsmith.plus.com/citroen2/

jeremy

[ellevie]

Thanks ! That's a really great site. It has some really useful information; the level of detail on the struts is really impressive.

[DLM]

If by any remote chance it's got anything more in it than the Citroen workshop manual (doubt it) I've got a copy of the Haynes all-singing-all-dancing-all-carburettor manual which includes that model. Can scan the section and email/PM it if necessary.

[ellevie]

Many thanks for your offer. The three docs carb1.pdf, carb2.pdf, and carb3.pdf are actually scans from the Solex & Pierburg Carburettors 1978 to 1992 by Haynes; I’ve got a copy myself.. The doc “Solex.pdf” is a Citroën doc, but it’s fairly short

I have actually solved the problem with my choke; the post was just for the benefit of anyone who might have the same or a similar start-up problem; it’s amazing how such an irritating problem can have such a simple solution - just a strip of PTFE tape. Initially I thought the pull-down diaphragm might have been perforated and need to be replaced. I didn’t even realize that the black plastic cover was a reservoir. I also considered installing a manual choke conversion kit which seems to be a popular solution to automatic choke problems. The choke is definitely fixed now, but I notice in this cold weather it takes a long time to reach normal idle revs. Still, this cold weather can't last forever.

[mnde]

Carrying on from my own auto-choke thread (http://www.bxclub.co.uk/forum/viewtopic ... 6&start=15) - I've had similar symptoms with my 1.6 petrol Solex carb.

I noticed this morning, which was particularly cold, that it will start first time and continue to run - as long as I don't touch the accelerator at all until it's been running for 30 seconds or so. If I use the accelerator while starting from cold, the engine will die almost immediately.

This morning I noticed that the engine idled at approx 1000 rpm when first started up, so the fast idle seems to be working, sort of... because this doesn't seem to be quite fast enough to get the engine warmed sufficiently quickly. While I was clearing the frost off the windows, the idle got progressively slower until after say 3 minutes it stalled before I could open the door and get my foot on the gas. Normal idle speed when hot is around 700-800 rpm. At the moment I can't get the air cleaner tube off the top of the carb because I'm lacking an appropriately sized allen key, so I can't do much about it until the weekend.

Mark.

[ellevie]

Hi Mark, it sounds as if you may well have the same problem as I had. It started up well but the revs gradually died down and it got to the stage where I had to dash back to the car after closing the garage door in time to stop it from cutting out. My fast idle is now about 1500. Even with the carb cover still on you should be able to check if the pull down is not operating properly. The lug at the top of the thin spring is directly connected to the choke spindle so you can lift this up with your finger a little bit to simulate the operation of the pull-down. In my case just lifting this a little bit brought the revs up like magic. Can you describe what your version looks like. Does it have the short spigot (about 2cm long by 3mm) which goes from the choke cover into a hole in the side of the diaphragm housing facing the cover ? Or does it only have a thin cover with no reservoir. Or does it look like the one in the Haynes manual with tubes everywhere? In my case it is a single stage pull-down with a reservoir built into the cover and by placing and removing my finger on the small hole I can activate and deactivate the pull-down and clearly see the little lug moving up and down by a small amount. If you look carefully you should also be able to see the brass rod along the bottom being driven by the diaphragm.

David.

[mnde]

Erm, I'll check and get back to you. It is a 1991 model so it has the B2C engine and should have the Solex 32-34 Z1 13928 carb. That extract from the haynes carb manual looks interesting, I'll have to print it out!

I noticed this doesn't list a fast idle speed for the BX16...

Mark.

[ellevie]

There appear to be 3 basic variations on the pull-down diaphragm (anti-flood capsule).

A thin single diaphragm module with no reservoir (therefore a thin choke cover).

A slightly thicker single diaphragm module with a hole on the side connecting via a short spigot to a reservoir in the choke cover. This is the one I have.

A much thicker double (2-stage) diaphragm module with a hole on top connected through a tube to the top of the reservoir in the cover. This is probably the one you have.

However complicated the arrangement, it still just determines the speed at which the pull down operates, at most a few seconds, but the more tubes involved the more places to look for a vacuum leak.

In the first place check that it really is a pull-down problem as described at the start of the post. I would then suggest removing the tube/s from the top of the diaphragm and temporarily blocking the hole with an airtight seal like a scrunched-up pellet of cling-film. If the pull-down operates then this would indicate that there is a vacuum leak associated with the tube/s otherwise it would suggest that the problem lies in the pull-down diaphragm mechanism itself, perhaps a perforated diaphragm.

David