Author Topic: 1920's barrel carby  (Read 10575 times)

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Offline Ian

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1920's barrel carby
« on: 10 Jul 2006 at 02:55 »
Hi all, I am still trying to sort out why my 2.75hp carby does not pull well at low revs. Have been playing round with it and have decided I do not understand some of the adjustments on it. The main one is the brass screw shown in the attached picture. It has a taper on the end which restricts flow but is not a normal needle valve. It has a tiny hole in the end which allows fuel through a drilling and then out holes on the side. Is this just meant to be a fixed restrictor or some sort of adjustable jet ? If adjustable then should it have a spring inside to keep it under tension ? It may ctually be the jet itself but there is a further jet from that chamber through to the slide. I am confused.



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« Last Edit: 10 Jul 2006 at 06:06 by alwyn »

Offline trevorp

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Re: 1920's barrel carby
« Reply #1 on: 10 Jul 2006 at 05:09 »
ian u dont have a breakdown so i can see inside it
ill explain basic and i mean basic carby theory
ok the smaller the carby the better low end power and larger carby gives more power top end hence the 2 barrel carbies on cars give u the best of both worlds
there should be 2 adjustments on these basic carbies and that is idle mixture which regulates either fuel or air to the small bleed hole on the engine side of throttle valve or slide
the idle mixture system allows u to get the correct mixture at idle once the slide or valve opens this system quickly stops working (some people belive adjusting this mixture screw alters fuel economy and power it doesnt)
Next adjustment is the slide or barrel idle stop screw this screw adjusts the slide height to be open enough for the engine to get enough air to idle, fairly basic this one
Above idle as in starting to pull away is governed by in the slide type carbie the needle valve and the slide cutaway
with the ilde system dying off
middle range is controlled by needle valve and a very slight amount the cutaway on slide
and full throttle is virtually only controlled by the main jet size
however to small a main jet effects the whole rev range more than an over size jet
adjustable main jets are very hard to adjust and easily knocked out of whack
im searching the web for a pic



Offline Ian

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Re: 1920's barrel carby
« Reply #2 on: 10 Jul 2006 at 05:26 »
Thanks Trevor - this thing is a bit wierd compared to a normal needle carby - runs as a two lever setup like a twin slide one so you adjust the mixture as you ride. The barrel rotates to vary air flow. It is this adjuster on the side I am confused about. I am worried that they way I have it set up it may be restricting fuel flow but undoing it doesn't make sense either !! Difficult to describe or picture but hopefully someone has one or documentation for them !!

In the picture you can see the brass barrel on the left which rotates with the throttle control and the mixture is adjusted with the other level and is a plunger going through the centre of the barrel thus restricting the cutaway.

Just added a photo of the valve I am referring to - there is a fine jet type hole in the tip.



« Last Edit: 10 Jul 2006 at 06:05 by alwyn »

Offline Doug

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Re: 1920's barrel carby
« Reply #3 on: 11 Jul 2006 at 05:32 »
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)

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2) 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:

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3) 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.

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My 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
« Last Edit: 13 Oct 2006 at 02:07 by Doug »

Offline Ian

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Re: 1920's barrel carby
« Reply #4 on: 11 Jul 2006 at 06:05 »
Thanks Doug - that is excellent information. Interesting - my carby is actually fitted with the float bowl facing forwards - guess it doesn't really matter - but does give access to removal of the main jet. If it was fitted the other way round one would have to remove the carby to do so. At least I now know that the jet is non-adjustable - and that it is a jet !! I poked some wire through the jet hole and think I may have had a partial blockage so i will see if it goes any better. What a silly place to put a jet so far away from the actual mixing chamber !! I will probably strip my rear tyre more quickly with the extra power !

Offline trevorp

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Re: 1920's barrel carby
« Reply #5 on: 12 Jul 2006 at 01:35 »
looks similiar to this but this has only one control
see here

Offline Ian

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Re: 1920's barrel carby
« Reply #6 on: 12 Jul 2006 at 02:16 »
Trevor, wish I had the bike to go with it !! (early FN)

Offline Doug

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Re: 1920's barrel carby
« Reply #7 on: 12 Jul 2006 at 03:01 »
The one illustration I mention above being a mirror image, is incorrect. I thought they had the cable pull off the outside face, but I see in Ian's photo it pulls off the back. So the whole card is just turned around (facing the wrong way), with the hot air tube pointing the wrong direction relative to the carb body. Probably the advertisement department in setting up the illustration did not know any better.

-Doug
« Last Edit: 13 Jul 2006 at 03:42 by Doug »

Offline alwyn

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Re: 1920's barrel carby
« Reply #8 on: 12 Jul 2006 at 03:56 »
looks similiar to this but .....

This is an interesting site that Trevor has linked - it is the home page of one Leon Mitchell from Adelaide in South Australia - it includes inter alia, information about some historical Australian built motorcycles - I have added a link in 'Other Links'. Thanks Trevor.

Alwyn
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Offline trevorp

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Re: 1920's barrel carby
« Reply #9 on: 19 Jul 2006 at 15:04 »
this is what u really need
See here
« Last Edit: 19 Jul 2006 at 21:33 by Dave »

Offline Dave

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Re: 1920's barrel carby
« Reply #10 on: 19 Jul 2006 at 21:36 »
Trevor,

The link wasn't working. Better check I've linked it to the right item.

Dave

 

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