Carburetor Ice

[Ian]

We had two 2.75hp Douglas machines out on a run this weekend in cold weather. Both suffered severely from carby ice. My friends one has one of the small sidedraft type carbys which can take a feed from the exhaust to keep it warm so we know the fix for that one. Mine has the oddball Douglas barrel type updraft carburetor which has no such facility. I was thinking of puting an inlet tube for it that picks up hot air from behind the muffler. Has anyone tried that ? Any othe ideas as to how to stop this ?

[trevorp]

by this ian u mean sticking throttle
Warming the air will work but decreases power but better than a sticking throttle
A cap full of metho in tank i mean a small capfull may help slightly and gets any water from condensation out of tank and can be burnt
My advice is put an air cleaner on it of paper type u can get them from these silly little monkey bikes that are around as these air cleaners are the only ones that are small enough for douglas carbies (sigh even a 50 cc monkey bike has a bigger carby than my t35)
i had these problems when i was playing around with water injection systems on a dyno and it used to ice the throttle slide to the carb body
And if that doesnt work maybe a tartan hot water bottle im sure it has a douglas part number

[Chris]

Carburettor icing is a common problem on Douglas models especially those with a single carburettor which requires excessively long inlet tracts. I am surprised if Trevorp has an icing problem with his T35 as that has twin carbs with the carbs bolted directly to the cylinder head. This is the only effective solution to the Dragonfly and those riders I know who do serious mileages in the depths of winter have converted their 'Flys' to twin carbs.
With older machines there are a variety of designs on various manifolds to pick up heated air from the exhaust system. There are tappings from one or other of the pipes from the cylinder exhaust  leading to a jacket on some carbs. With respect to the Douglas alloy barrel type carburettor usually fitted to the later machines around 1925/26, there are two different designs of pick up pipe depending upon the type of exhaust fitted. These are also cast in aluminium one looks like a Banjo with large round head corresponding with the circular cover of the cross tube type of exhaust sytem. The other is a down tube extension with a wrap around casting at the base designed to pick up its air supply from around the pipe travelling horizontally to the rear of the machine. Unfortunately, I have only seen these castings in period illustrations or on particular bikes. I have never seen one offered for sale

[Doug]

Ian,

Here is a 1926 catalog image of a carb inlet heater, which Douglas called the "Special fuel economy heating pipe". I am sure this is the one Chris calls the "banjo".


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Also some pictures of the machined but as yet rough castings. Note the air inlet seems to be a slot on the front face, marked X. There is no communication to the rear face, which is just a heat transfer surface.


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A second style is shown below, I think the second type mentoned by Chris. I do not recall a catalog illustration showing this type myself, but it would seem to be for exhaust systems where you do not have the central silencer can directly under the engine, rather a pipe(s) heading to the rear of the bike where it may, or may not, enter a silencer. In this example I would think that (again a rough casting) needs additional machining, to punch a hole or two from the portion that wraps around the pipe into the cored passage leading to the carb. Though why it is not already cast in, it would have helped support the existing core, I have no idea.


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I have only seen pictures of these, if I ever saw an actual one, it has escaped my memory. 

Douglas kept on with hot air for the carb. Many of the later twenties and thirties models with the carb above the engine, had a inlet tube for the carb that drew air off the rear cylinder. Some models had jacketed manifolds with exhaust heat, or a 'hot spot' in contact with the crankcase to transfer some heat. So it is part of the Douglas tradition! Rather than a hot water bottle, you could lag the inlet manifold with an old tartan kilt.

-Doug

[Ian]

Thanks for all the suggestions - I think the banjo type one would be the way to go - or a different carburetor. I have never been impressed by the barrel one. Chris, you mentioned that it is from 25-26 seasons ? Mine is a 23 model and I have always persisted with this carby because I thought it was original - not the case ? I would be delighted to convert to a more "normal" style as I find this one gives very little pull at low revs.

[trevorp]

Sorry Chris i have no trouble with my t35 most of my experience is on a dyno testing alternate fuels on vehicles
Ian just one thing to take into account these heating systems are mainly for British weather i know in Australia we have the same conditions especially inland in winter but in summer these heating devices make your bike hard to start after running for a while they boil of the fuel in the float bowl which re condenses in manifold or near valves and has a tendency to flood engines with fuel.
i would be thinking of something along the lines of the heater but make it removable for summer riding
And i think Doug has hit the nail on the head with the kilt as they were the temperature gauge for riding a Douglas, ask Ian to ride around inland Aussie in winter with a kilt on and he will need more than his carby warmed

[Ian]

I know what the kilts are like Trevor - I used to have to wear one at school for cadets !!

The conditions on the weekend were perfect for carby ice - it was so bad the bikes actually stopped on a number of occasions through complete blockage. High humidity, cold temps, etc. The temperature does not actually have to be particularly low - as a pilot I know we get worried about carby ice when the temp is below about 20 C. My intention with mine is to remove whatever I do in summer as I don't have problems then.

I think the other issue we have here is that the fuels used in Australia are (so I am told) quite dense and suited for hetter climates. When it is cold they do not vapourise well. This made me think that with the long, cold inlet manifold on the Douglas that we may be having some problems with that - thus reducing the power in cold weather. The OC that I am rebuilding has a full double skin manifold with the valve to regulate exhaust gas heating - much better !!

[Chris]

Thanks Doug for finding the illustrations and posting them. The wrap around type does have a full bore hole from the wrapping section straight up to the carb. It also has a hole for a screw and there is a shaped clamp secured by the screw and a nut so that the assembly is clamped around the exhaust pipe.
I was incorrect in saying that the barrel type carburettor was fitted 25/26. It was actually fitted first in 1924. However, I believe it was only fitted to the CW Model. This model was only produced in 1924 , 25 and 26 and there appears to have been very few in 1924, Hence my earlier statement. Illustrations of the 1924 CW show the tubular across the frame exhaust with the "Banjo" type air collector. The 1925 and 1926 CW show the exhaust system running back to a silencer at the rear. Commonly these have an angled air collector taking air from just above the exhaust pipe but there are also examples of machines with this type of exhaust having the wrap around collector and my CW is one of these. I have illustrations of the TS model of 1924 showing an horizontal draft carburettor which looks like an AMAC. This is what I have on my 1923 TS but I cannot vouch for its originality. The history of my machine includes the statement that in 1936 it was purchased without a carburettor and a suitable second-hand Douglas unit was found and fitted. Another carburettor, a  Bink's 3 jet, also came in the box of bits with the machine but looked too large although these carbs. were fitted as original equipment to some models in 1920/21.
It is likely that Ian's machine should actually have the brass  horizontal draft carburettor but that said, the barrel carburettor is quite a good piece of kit providing it is not leaking too much air at the rotary barrel. Nice examples of the earlier carburettors are also becoming very scarce.

[Ian]

Thanks Chris - I think the air past the barrel may be the problem - mine has a bit of sideplay

The brass carby you refer to - what make was it - Douglas or an OEM ?

[Chris]

There are two different brass carburettors (nickel plated) that are very similar. (ignoring the earlier designs used on veterans). One is updraft and one horizontal. They are identical inside in terms of the slides and the top hat assembly with its cable adjusters. I believe they were made by AMAC but the updraft version is often referred to as a Douglas AMAC. I really don't know whether AMAC made this carb. solely for Douglas or whether it was used on other makes of bike. The problem with Douglas is that very rarely is a particular carburettor (or magneto) specified for a model. The usual comment is that items of good make are fitted. One is drawn back to the conclusion that with their regular financial problems, a change of make is probably more influenced by running out of credit with one supplier than any desire for a better or different performance. When supplies dried up completely, then they were forced to make their own.

[trevorp]

if someone can give me a model number size and flange i will add them to my ebay searches

[Ian]

Chris kindly provided these photos of the two brass AMAC style carbys that were available.



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[gsx1100s]

I was searching the forums for advice on my Amac carbie. I have found that my carbie must be an early model one as it doesn't seem to fit the bill anywhere. the picture above is the closest to it ( the silver carbie). My carbie is brass and is bolted to the manifold on the side not underneath . it also has two cylindrical projections from under and either side of the air intake. Any suggestions at to what these are ? And are there any detailed drawings of this carbie I may be able to find ? I would like to know whats missing from mine. 

Thanks in advance,

Michael

[Ian]

Michael, it may be a double skin one where it can have hot air takeoff from the exhaust system which keeps things warm - the projections would be an inlet and outlet for the warm air. The exhaust system would have a place where you can attach a pipe for this. If you have a look at the closeup photos of Kevins 1920 Model W on the photo identification pages you will see what I mean.

[gsx1100s]

Many thanks Ian that has solved that riddle.
Wouldn't the hot air being forced back into the carbie be void of combustable oxygen? Or is it not scavenging the air for use , but circulating it?
Can the carbie be run without using the pipes? say if one were to put a blank on the opening?

cheers Michael

[Ian]

If the carby is the same as Kevins then the air from the exhaust only circulates round it - doesn't get sucked back in. Without the pipes it will run OK on hot days but will literally freeze up on cold humid days. Maybe put up a pic ?

[gsx1100s]

I would happily send a picture , but don't own the equipment to do it, sorry  , but very many thanks for the information.

regards Michael

[Alan Cun]

Hello All     The silver side draft.  Go to model by model childboards    You will see the Carby mouth piece that looks like an old brass door knob with a dimple. At the back circumference is a series of 1/4 inch holes, now in theory the warmth of the motor should be drawn through. Does this happen not much, anyway  I dont think so.   The old timers always reckoned the alloy Doug Rotating Carb was a bit of a flop. My old mate Brian never had a problem with icing on the 60 MPH plus think he cheated with a side bowl Amal.  Al