34DMTR Carb Idle Circuit
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  1. #1
    Tadpole
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    Default 34DMTR Carb Idle Circuit

    Morning all,

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    So after a recent carb issue on my CX I gave it a quick refresh (diaphragm, gaskets, needle and seat, good clean out).
    I didn't want to start from scratch tuning-wise, so set everything as it was before I took it apart (as the car ran, was just very hard to start, rich and rough).

    However now the car refuses to idle, it will only run (barely) with the throttle wide open, and is 10x richer than before, regardless of the position of the idle mixture screw.
    I'm too young to have dealt with carbs much so have only touched the basics so far (idle mixture screw, idle speed, with little change). Do any carburetor whizzes have any suggestions before I go calling in a specialist? Could this be a jetting issue I've created?

    (it's a '78 2400 Cmatic with a Weber 34DMTR39/350. So far I've ignored the mechanism that compensates for the torque converter as I understand it only works when a gear is engaged)

    Happy holidays everyone!

  2. #2
    Fellow Frogger
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    There are a number of possible reasons and you are really best to work though the assembly procedure in the manual. The jets are fixed so not adjustable, but could be blocked or the incorrect spec - check the marked numbers against the book.

    Is the fuel free of water. It can enter via the cap seal and collect in the bottom of the filler pipe until some if pushed into the tank. You can't drain the tank via the filler, but you can drain the neck and see if it contains any water. You can also disconnect the carby feed hose, direct it to a container and crank the engine to see what comes out. In case you weren't aware, there is a connector for the stater solenoid near the battery +ve terminal with a red tag from memory. You can crank the engine single-handed with ignition off by touching it to the battery +ve. Be careful with these steps as fuel + spark = danger! That will confirm the pump is working and what it is delivering.

    There is also an idle solenoid. It's the small cylinder at the back with one spade connector and it won't idle without it connected and energised. It's a plunger with a small spring for shutoff.

    Given you changed the float valve, did you adjust the float clearance? If not the float chamber may not have fuel at the correct level, which is a fundamental setup point. It's worth pointing out here that it is possible to dress the valve in a lathe and reuse it if availability is a problem.

    The jets are fixed, but you can alter the flap settings. Depending on what you did, you may find the flaps are not operating as intended. The manual will provide the minimum clearance settings and you should observe those.

    If it has A/C, you have a vacuum step-up valve to increase the idle with AC running. It has a screw for adjustment and is a vacuum operated. Turn the A/C off and disable that for the moment. It has a spring inside to shut it off when there is no vacuum. I think the vacuum solenoid for it is on the RH wheelarch near the coolant reservoir.

  3. #3
    Tadpole
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    Much obliged David, the float level was the magic fix. I didn't consider that the clearance would change, it's much happier now. Time for some 98 and some fettling

  4. #4
    JBN
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    From memory, the mechanism that operates the idle for C-matics is linked to the carby via a lever. My wife drove from Kings Cross in the centre of Sydney to Sutherland in the south with this lever off and the engine idling at a few thousand revs. Fortunately both God and the traffic lights were on her side and she wasn't in an accident.

    John

  5. #5
    Tadpole
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    Yep, unfortunately mine's pretty worn so a previous owner has disconnected the system, putting the car into gear drops the rpm but not enough to stall it. I imagine if I had the A/C hooked up it'd stall but it was such a pain to work around I took off the compressor.

    Now the car's a bit happier (and not leaking everywhere) it might be time to sort out some of these peripheral systems. Dealing with the mess of leftover anti-pollution systems will be next I think.

  6. #6
    Too many posts! JohnW's Avatar
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    If you find you really need another carbie, shout. I seem to have several. My AC idle step up works but I'm not sure the C-matic step up has ever worked. It's never been prone to stalling and I only found out about that system after ten years of ownership. You can't really tell the idle speed from the tacho, not least because it starts at 1000 rpm. Mine has always jumped around a wee bit at idle, intermittently, for example when turning AC on and off - no doubt an earth somewhere at the back of the instrument cluster. Until it stops working I'm not chasing an intermittent!

    For my money, a C-matic is a nice car. It allows maximum wafting. I reckon I'd like an automatic too, suiting the CX character. I drove a RR Silver Shadow the other week and it provides another approach to wafting. Apart from getting in and out, I think I prefer the CX but the Rolls was a nice drive in its envelope. I think it costs its owner about the purchase price of a CX every year for servicing.
    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
    Tadpole
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    Thanks for that, I'll remember that if it gives me any major grief. For chugging around the yard it seems alright, when I bleed the brakes I'll be able to take it for a proper run and see how it copes under load.
    My tach is hopeless too, there's a 3-4 second delay between rpm change and it showing it, though the speedo woke up today.

    Mine's certainly been getting nicer over recent months (I bought it with 0 fluid in the transmission so we'll see how that goes, another story). I can definitely see the appeal, it's definitely got the edge over the RR in weirdness, which was the whole reason I bought it. Looking forward to actually driving it one day

  8. #8
    Too many posts! JohnW's Avatar
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    Shout anytime. They are quite charming once working properly. Yours sounds as if it has some arthritis from lack of use - could be sticky bearings in the tacho drum too, although the earths and instrument panel printed circuit system aren't too robust.

    They are HOT cars in summer. Big heat source, inadequate insulation and they never got the ventilation sorted well. You can check whether the outlet vents are open or taped over, underneath ally panels immediately behind the rear door windows. I found one open and the other taped up (and removed the tape promptly. There are any number of posts here but I think Shane might have a rare example of a cool CX thanks to stripping the interior and insulating the shell properly. Our AC works correctly but it still isn't really enough.

    I've no experience of it, as ours was fixed before we got it, but beware of the C-matic transmission clutch slipping. There's an internal seal that fails apparently and the clutch doesn't get enough hydraulic pressure to remain engaged properly. Again, someone else like Shane can provide proper advice I imagine.

    Cheers
    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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