Ian,
O.k., so you are bound and determined to suffer like a true Douglas enthusiast, by persisting with the diabolical Douglas barrel carburetor!
There are several iterations of the barrel carburetor. I am familiar with the final version as fitted to several unfortunately blessed models of the mid thirties. I am not certain of the chronology of the earlier versions but I believe it goes something like this:
1) Single lever operation, just the barrel valve is cable operated. As featured in J.J. Barter's patent application of 1921. The main jet is adjustable via a large numbered dial. (See next image)
Larger view2) Two-lever operation. One cable rotating the barrel air valve and the other rotating the adjustable main jet, which still has a finger adjustable dial. As seen in Multi-Cylinder Engines, author and date unknown (I only have a few photocopied pages from this, so if someone could enlighten me?) But it mentions it as a subsequent development to type 1 (which is logical.) The "Multi-Cylinder Engines" as mentioned above, showing type 1 and 2:
Larger view
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Larger view3) Two lever operation. One cable rotating the barrel air valve and the other withdrawing the cylindrical valve inside the barrel. Featured in catalog illustrations of 1924. This is the version you show. The main jet is not adjustable, but can be replaced by ones of different sizes as mentioned in the 2-3/4hp handbook of 1912-25. Despite the hot air pipe pointing to the rear, I suspect this image to be printed in reverse. (See next image)
4) Single lever operation. As used on some 1934-35 models, perhaps as early as 1931. Barrel rotated by cable. The main jet is not adjustable, but can be replaced by ones of different sizes. Idle bleed needle valve adjustment. (See next image)
The main jet on yours and later barrel carburetors uses the same fine thread as the earlier carb, making it superficially look as if it were meant for adjustment. But it is not the case; the steep taper on the end is just to provide a seat to seal. The earlier carbs that did have an adjustable jet had a much more gradual tapered needle tip (no hole) as one would expect.
The design of the type 4 main jet is identical to yours the type 3, with the cross drillings, large fine pitch thread (probably using the same taps left over from the adjustable jet) and large adjustment slot buggered up by misguided owners trying to 'tune' it. The 2-3/4hp handbook mentions this slot is extra wide so that the jet can be removed with the edge of a coin, to change over to one of the spare size jets. It also mentions it is important to make a good seating with the taper on the end, or "bad running will result." I have not seen any info what optional jet sizes were available. The 1934 version uses a �0.046� main jet orifice, and the fuel nozzle has a 0.094" orifice; but this is for a 0.900" choke, 600cc o.h.v. engine and will be far too large for your 2-3/4hp model. On the type 4 there is a small hole to the outside that joins up with the cross drillings of the main jet. I can not tell if they are present on the type 3, but suspect it is. It is visible in a cutaway illustration of a type 2. I suppose the idea was to emulsify the fuel slightly after passing through the main jet, but before passing through the fuel nozzle. This is just speculation; there is no mention of the vent hole or purpose in Douglas literature that I have yet seen.
The jet at the barrel valve is not a jet at all, but what Douglas called a fuel nozzle. And that is what it is; it spits and sprays globules of raw fuel into the air stream. Most of which is drawn up into the engine, the rest dropping out to the ground. Probably the true purpose of the heater muffs fitted to the earlier barrel carbs was not so much to supply warm air to prevent carburetor icing, but to direct the dripping fuel onto the exhaust manifold where it would be vaporized and so consumed by the engine! But the handbook tells one to remove the hot air pipe in the summer, so I suppose that is just slander on my part!
The type 4 is fitted with an idle circuit as well, more of an air bleed above the barrel valve, adjacent to a notch in the valve that passes the air when the valve is shut to it minimum opening. There is some question as to how this was supposed to work originally, as just bleeding air down stream of the valve is not going to help the idle much unless you wished to lean it out further. It had practically no effect on the engine until I extended the drilling from the float bowl to connect with its passage. Then it operated much as a normal pilot needle adjustment and I believe it was just a mistake in manufacturing. But I have never had the chance to examine another one of these carbs to see if it had the same fault. In the following illustration,
A indicates the well for the main jet, and
B the location of the idle needle bleed. None of the gallery plugs are shown in this drawing (it was made to manufacture replica bodies), so you will have to use some carb knowledge and imagination. It is not clear to me if the earlier carbs have this idle circuit, but being multi-lever 'non-automatic' I suspect not; nor do I see external indications of such.
Larger viewMy personal experience with the type 4 is that it is not very good at all for low speed running. I suspect the atomization of the fuel is not very good, as indicated by the fuel dripping out of the choke at idle. And by choke I mean the main bore of the carb, and not in the American sense of a restrictor for cold starting; which the Brits called a strangler. From about 3/4 to wide open the carb works rather well! The low speed adjustment did dramatically improve when I drilled through the gallery as mentioned above, but it never became what you would call civilized.
-Doug
