CX S1 A/C and C-matic carburettor fast-idle circuit
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  1. #1
    Tadpole
    Join Date
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    Icon7 CX S1 A/C and C-matic carburettor fast-idle circuit

    I have a '79 CX 2400 C-matic w/ carburettor. A friend of mine has the same car - but w/ 4-speed manual and A/C. (The funny thing is that both cars are green metallic AC 538 "Vert Dryades"- "fairy green" - a quite rare colour only offered for the model year 1979). Both cars was sold new in France and we aquired then in the Netherlands about 2 years ago.

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    I've found out that we both have the wrong carburettors! CX 2400 Models with A/C or C-matic (or both) should from model year 1977 be equipped with a Weber 34 DMTR 35/350 carb. (34 DMTR 25/100 or 150 for CX 2000 and 34 DMTR 28/100 or 300 for CX 2200 with options as mentioned).
    It turned out that my CX was fitted with a 34 DMTR 35/250. The latter is for the basic CX 2400 w/o A/C or C-matic in '77 and early '78; from mid-'78 CitroŽn used a Solex 34 CICF 161 which what was we found on my friends car. The difference between these two carbs and what should have been there is that the std. carb lacks the "fast-idle" circuit.

    The "fast-idle" circuit consists of a vacuum-operated device on the carb and an electrical operated vacuum valve mounted close to the right front wheel sphere. It opens or closes so that vacuum from the engines inlet via a rubber hose can or cannot be applied to the membrane in the "fast-idle device". I've managed to get hold of such an carburettor now, see attached photo w/ captions.The other photo shows how the tubes and vacuum electro-vale is connected. (Photo supplied by courtesy of Manabu Kosuge in Japan and is taken from his 1980 CX 2400 Pallas C-matic LHD).

    CX S1 A/C and C-matic carburettor fast-idle circuit-weber_w_fast-idle-device.jpg CX S1 A/C and C-matic carburettor fast-idle circuit-engine-bay.jpg CX S1 A/C and C-matic carburettor fast-idle circuit-vacuum-suction-spigot.jpg

    The philosophy is that when you put the C-matic gearbox in gear the drag of the torque converter will lower the engine's r.p.m., and the "fast-idle" circuit will counteract that by feeding the engine with a little more air/fuel mix. The same applies to the A/C models, switching on the A/C activates the relay that controls the vacuum valve that in turn counteracts the drop in idle r.p.m.
    I can confirm that it's hard to manoever the CX while parking in a garage-house w/o any "fast idle"-device, the idle is very low when "creeping" and the engine will easily stall. Compensating for low r.p.m. by giving more throttle is difficult as your right foot will be on the brake pedal and sudden "leaps" caused by attempts to keep the engine from stalling is not desirable...

    The CitroŽn p/n for the vacuum valve is 5-426-080 and is made by Pierburg. This part seems to be a "dark horse" - it is "unobtainium" at all the usual CX parts sources (not even mentioned) and I have so far found no mention of it in this forum too. I know Aussie models may have somewhat different specs (RHD and 2nd rear mounted A/C) but I reckon that Aussie carb-models w/ C-matic and/or A/C also must have a fast-idle circuit.

    (And there seems to be a lot of CX's in Australia both with C-matic and w/ A/C. In Europe it seems that if there was perceived any trouble whatsoever with a C-matic box the "cure" was to replace it with a manual, typically a 5-speed. And A/C was a luxury option and not a bare necessity as in Australia. So there are very few CX S1 around w/ A/C and/or C-matic here, at least carburettor models).


    Anyway - I take my chances and ask you guys "down under" if anybody has any tips on where to find the "Vacuum electro-valve" p/n 5-426-080? And also the little drum-shaped white air-box next to it - p/n 5-491-311. Or suitable replacement parts for these. Thank you for any feedback!

  2. #2
    Tadpole
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    Hello, I'm new to this site. Thank you for accepting me.
    I owned a 1978 CX 2400 with the C-Matic 'box. It was a 12,000 mls car when purchased. I can confirm that running-wise, during low speed maneuvering, it performed flawlessly and without issue. So that's what they were like when nearly new.

    It is a long shot, but try eBay and trawl the Chinese copies of Weber carbs. There probably will be other versions with a similar vacuum fast-idle feature?
    The range of stuff that they 'replicate' is amazing. They are really cheap too. Quality is OK.
    My sister has an obscure version of a Golf cabriolet. We managed to get a new Weber carburettor for it from China and it works really well. Good luck.
    Sorry, I'm an idiot. Didn't read you post properly.
    Have a look at the Rover 216 (Honda Civic based) this model did an 'idle-up' when you activated the heated rear window. This may be adaptable to your requirements. If you cannot find what I'm on about, please ask. Thank you.
    Last edited by tobytronic; 24th April 2018 at 10:13 AM. Reason: Poor comprehension skills.

  3. #3
    Real cars have hydraulics DoubleChevron's Avatar
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    Most parts cars in Australia should have these parts .... However, the carbies are extremelly worn (the spindles eat there way through the carby body ... and weber designed the carby, so if you try to bore the area out ... it's break up). The diaphrams are generally dead in both the idle advance on the side of the carby, and the vacuum switch.

    Basically anything that has lived under a (very hot) CX bonnet in Australia for 30+ years and is made of rubber..... Isn't going to be much good sadly. If by some miracle you find working parts, they will likely fail within a short time once pushed back into service.

    NOS parts are the go. Does CX Basis have anything The problem with buying aftermarket parts that contain rubber ... is they almost all seem to be complete garbage ... and will simply not last, not even for a short time.

    With the vacuum switch ... there will be hundreds of varieties on ebay. Just pick one you think you can easily fit up (ie: hose size etc). The high idle attachment to the side of the carby. You might need to get it re-diaphragmed. There is companies that do this. The quality should be a lot better than replacements (I'm on the land rover forum too. and there is people that literally have fitted dozens of vacuum advance units to there cars ... and they fail with days/weeks. When pulled apart the manufacturing is so poor there is sharp edges in them that pierce the diaphragm as soon as the vacuum is hooked up.

    Where on earth can we get quality replacement parts these days

    seeya,
    Shane L.
    Last edited by DoubleChevron; 24th April 2018 at 12:31 PM.
    'Cit' homepage:
    Citroen Workshop
    Proper cars--
    '85 Series II CX2500 GTi Turbo I
    '63 ID19 http://www.aussiefrogs.com/forum/citro%EBn-forum/90325-best-project-car-you-have-ever-seen.html
    '72 DS21 ie 5spd pallas (last looked at ... about 15years ago)
    '78 GS1220 pallas
    '92 Range Rover Classic ... 5spd manual.

    Yay ... No Slugomatics


    Modern Junk:
    '07 Poogoe 407 HDi 6spd manual

  4. #4
    Fellow Frogger
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    I used a nippondenso vacuum electrovalve to control an additional heater tap in my long-deceased CX. They were often used on the inlet manifold for emission control systems of many makes and sometimes had an inbuilt filter. It would do the same job as the Pierburg valve for the AC step-up control, but won't look the same. Replace the white drum with a standard in-line fuel filter. If you have factory AC, the wiring should already be present for the electrovalve.

    Cars in our market had an additional accumulator. Yours may have only two. You should still have some steering assistance immediately after switching off the engine, but only for a very short time as it is a large consumer. If the steering is heavy, it is always possible there is some wear in the control unit components that track the difference between the rack and wheel input positions and operate the ram valve. There are 4 balls that rest in two wheels and their relative movement operates the valve. If worn, you will always be fighting the rack initially until you have moved the steering wheel enough to overcome the wear and make the valve move. You'd notice this most when making small movements at low idle. It can also become stuck at full lock and possibly exhibit some hysteresis.

  5. #5
    Real cars have hydraulics DoubleChevron's Avatar
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    if his car is a carby car... its likely it has a 7cylinder ds type pump. They barely keep up with the demands of the power steering. They are fine in manual cars as you just rev the motor slightly to speed the pump up a bit. In the C-matics and autos you'll be fighting the steering. Our old cx diesel wagon was the worst for this. It would idle at probably 400rpm .... and almost always ran out of assistance when trying to park it.

    seeya,
    Shane L.
    'Cit' homepage:
    Citroen Workshop
    Proper cars--
    '85 Series II CX2500 GTi Turbo I
    '63 ID19 http://www.aussiefrogs.com/forum/showthread.php?t=90325
    '72 DS21 ie 5spd pallas (last looked at ... about 15years ago)
    '78 GS1220 pallas
    '92 Range Rover Classic ... 5spd manual.

    Yay ... No Slugomatics


    Modern Junk:
    '07 Poogoe 407 HDi 6spd manual

  6. #6
    Too many posts! JohnW's Avatar
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    Interesting. I have this system working.

    I have quite a few good looking carbies so must pull them out and check their condition - they've been in boxes for many years. I could post one to Norway quite easily and it wouldn't cost a fortune. I take the point about tired diaphragms of course.

    Regarding idling, I have this vague memory of there being another circuit to increase the idling speed with a C-matic when it is in gear not in neutral??

    Shane, please remind me what the alternative hydraulic pump is, to get more delivery? I'm sure there was a post about this years ago.
    JohnW

    Renault 4CV 1951
    Renault R8 1965
    Renault Scenic 2005 (wife's)
    Renault Scenic 2007 (mine)
    Renault Scenic 2006 (daughter's)
    CitroŽn CX Pallas 1980

    National Co-ordinator, Renault 4CV Register of Australia

  7. #7
    Real cars have hydraulics DoubleChevron's Avatar
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    Quote Originally Posted by JohnW View Post
    Interesting. I have this system working.

    I have quite a few good looking carbies so must pull them out and check their condition - they've been in boxes for many years. I could post one to Norway quite easily and it wouldn't cost a fortune. I take the point about tired diaphragms of course.

    Regarding idling, I have this vague memory of there being another circuit to increase the idling speed with a C-matic when it is in gear not in neutral??

    Shane, please remind me what the alternative hydraulic pump is, to get more delivery? I'm sure there was a post about this years ago.
    its just a later 5cylinder pump. later CX's use them as does every car made after the mid 80's

    seeya,
    Shane L.
    'Cit' homepage:
    Citroen Workshop
    Proper cars--
    '85 Series II CX2500 GTi Turbo I
    '63 ID19 http://www.aussiefrogs.com/forum/showthread.php?t=90325
    '72 DS21 ie 5spd pallas (last looked at ... about 15years ago)
    '78 GS1220 pallas
    '92 Range Rover Classic ... 5spd manual.

    Yay ... No Slugomatics


    Modern Junk:
    '07 Poogoe 407 HDi 6spd manual

  8. #8
    Too many posts! JohnW's Avatar
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    Thanks Shane. I ought to get one......
    JohnW

    Renault 4CV 1951
    Renault R8 1965
    Renault Scenic 2005 (wife's)
    Renault Scenic 2007 (mine)
    Renault Scenic 2006 (daughter's)
    CitroŽn CX Pallas 1980

    National Co-ordinator, Renault 4CV Register of Australia

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