Carburetter Identification

[Paul]

Hi,
Can anyone identify the carburetter fitted to my M28 602 cc engine. I have no idea of the history, so the carbureter may not be original. It's a twin choke solex and has the numbers '27 B019 01' stampled on the float bowl. There are also two casting marks '24' and, I think '21' either side of the 'SOLEX' marking. Could the carb be a 26/35, or some carb taken from some entirely different engine? Also, is it a CSIC or a SCIC; whatever they stand for?

I need the information to check that the correct jets are installed as I am having some problems with a flat spot when the accelerator is initially depressed. Some of that may be due to a the advance retard being slow to advance until the engine speed appears high, but I thought I'd check the jets are correct first.

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Attached, hopefully are a couple of photos. Thanks.

[callum]

Paul,
From memory, it's a carb from a Dyane - slightly different choke sizes. The SCIC refer to emmissions controls etc.

A flat spot upon acceleration would lead me to check the accelerator pump first. These carbs can suffer a flat spot bwteen first and 2nd choke, which can be rectified with a different primary jet, but that doesn't sound like your problem.

Take the rubber goose-neck off the top of the carb and pull the accelerator pump lever - you should see the fuel squirt down the first choke.

Also, the choke pull-off device is missing. Either replace it or ensure that the hole is blanked off.

Rgrds
Callum

[Ken Hanna]

Paul and Callum,
there was no such thing as a carburettor specific to Dyanes, since the change from the earlier version of the 26/35 carburettor (with 21 & 24mm throats) to the later type (with 18 & 26mm throat diameters) was made by Solex, not Citroen.
Not too sure of the date when the change was made, but ECAS quote 1982 for the final version, with its awful 'choke pull-off' device.

Also, 'SCIC' identifies those versions of the carburettor fitted with a throttle closing dashpot, required for vehicles using a centrifugal clutch.
'CSIC' identified the standard version.

A 21-24 carburettor would not have been fitted with a choke pull-off device ex-works, however it's not unknown for carburettor lids to have been swapped around.

In addition to checking if the accelerator pump is working correctly, it's also worth checking whether the closing position of the secondary barrel needs adjustment.
Since there were different locations for the adjusting screw, a couple of 'side on' photographs would be handy.

If the operation of the centrifugal advance mechanism is sluggish, you should check that the backing plate behind the points box hasn't been distorted due to over-tightening of the 2 x M7 bolts securing the points box, bringing it into contact with the bob weights or their mounting pins.
I've lost count of the number of back plates on which I've had to grind or file down the edges so that they will sit flat in their housing.

Ken

Paul,
From memory, it's a carb from a Dyane - slightly different choke sizes. The SCIC refer to emmissions controls etc.

A flat spot upon acceleration would lead me to check the accelerator pump first. These carbs can suffer a flat spot bwteen first and 2nd choke, which can be rectified with a different primary jet, but that doesn't sound like your problem.

Take the rubber goose-neck off the top of the carb and pull the accelerator pump lever - you should see the fuel squirt down the first choke.

Also, the choke pull-off device is missing. Either replace it or ensure that the hole is blanked off.

Rgrds
Callum

[Paul]

Ken,

I took the caburetter off the engine and stripped it down. The hesitation with acceleration is probably due to the wear on the roller for the pump lever; the roller is worn completely through, so there is a lot of lost movement (see the photo). So a new roller is in order.

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With regards to the adjusting screw for the second throttle valve, I've attached a photo.

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Comparing the jets in my carburetter to the sizes quoted in the Haynes manual, some of my jets appear to be smaller; my jets are as follows:

1. Primary main jet 120 (Haynes state 125).
2. Secondary main jet 70 (Haynes state 75 or 82.5).
3. Primary air correction jet 1F2 (Haynes state 1F1).
4. Secondary air correction jet 2AA(Haynes state 2AA)
5. Pump injector not removed in case I snapped the jet.
6. Idling jet 37 (Haynes state 50 or 40).
7. Needle valve seat not checked.

The air filter is a paper element from a Petter 600 cc diesel engine, so I'm thinking that it'll be on a par with the stansard Citroen filter. The exhaust is standard up to the exit from the silencer under the gearbox, whereupon it discharges into a second silencer, origins unknown. The upshot is that the intake and exhaust system are probably close to standard, so standard jets should suffice, or am I wrong.

I haven't got round to checking the backing plate yet; that's next on the list.

Regards Paul

[Ken Hanna]

Paul,
there were minor changes made to the jetting over the years of production, with identification of each variation made possible by a stamped alloy tag fastened to the carburettor by one of the lid screws.
Most of those tags have been discarded by now, but the details you've given for your carburettor corresponds with Solex's code for the most recent 21/24 carburettors, whereas Haynes' info covers that for earlier versions.

In general, those 21/24 carburettors which were fitted with a 120 primary petrol jet will benefit from an increase to 125, with that change being primarily to cater for the 'leaning' effect of the additives containing oxygenates used in unleaded fuel.

In detail, a 37 idle jet is close enough to a 40 not to make any significant difference.
The adjusting screw for the closing position of the secondary butterfly on your carburettor still has the 'anti-tamper' cap intact, so the top of that needs to be snipped off and the screw backed off about one turn.

If you cannot track down a replacement roller for the accelerator pump lever, the cover & lever from an 18/26 carburettor can be substituted, but make sure that spring inside the chamber doesn't escape...

Ken

[Paul]

Ken,
Thanks for all that information. I've ordered a new roller and spindle, a 125 primary jet, accelerator pump diaphragm and spring plus some gaskets from ECAS. So in a few days time I should have the engine running. Thanks again.

Regards Paul

[Paul]

Ken,
Sorry, I forgot to ask. What is the logic behind backing off the adjusting screw for the closing position of the secondary butterfly by one turn? I was always under the impression that this setting was set at the factory and shouldn't be disturbed.

Regards Paul

[Ken Hanna]

Paul,
just the realisation that, after many miles of use, the original setting is no longer effective because the butterfly will have worn away its seating in the bore.
Experience has shown that one turn back on its stop screw usually restores the seal.

If you want to put it to the test, try pouring some cheap engine oil into the secondary barrel with the engine warmed up and ticking over.
If the oil rests on top of the butterfly, there's no problem.
If it's drawn through straightaway, adjustment is needed.

Done two of these this week, both carburettors now have much more positive response to adjustment of the idle mixture screw and the throttle stop screw, as expected.

Ken

Ken,
Sorry, I forgot to ask. What is the logic behind backing off the adjusting screw for the closing position of the secondary butterfly by one turn? I was always under the impression that this setting was set at the factory and shouldn't be disturbed.

Regards Paul

[Paul]

Ken,
The carburetter with new 125 primary jet, accelerator pump roller and float height adjusted is back on the car.

I had to do another repair. The choke lever and its operating pin were near enough worn through, so the choke was still closed by a few degrees (probably insufficient to worry about). I made up a triangular plate with the correct curve, then Araldite'd it to the choke lever. The new plate now bears on an unworn surface of the operating pin (see photo).

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I didn’t back off the secondary throttle screw because I sprayed brake cleaner into the venturi and none leaked past the butterfly. I know it’s not quite the same as engine oil, but the brake cleaner is as thin as gnat’s pee, so that seemed good enough for me. However, I did remove the anti-tamper cap in case I need to adjust in the future.

Anyway, the upshot is, the acceleration is improved no end, tickover is more stable and the choke seems more sensitive.

A good result for me so thanks.

Regards Paul