Author Topic: Re: DT/SW5 Carbs  (Read 539 times)

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Offline Buzzie

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Re: DT/SW5 Carbs
« on: 05 Mar 2019 at 22:26 »
Some of you may have my seen thread about my DT5 engine project. (https://www.douglasmotorcycles.net/index.php?topic=7270.msg28115#new )

The project has been brought to a, hopefully temporary, halt by what looks like carburetion problems. I have started to look into this and can find no obvious reason why the carbs arent working properly. I am finding it difficult to make head nor tale of the various bits of information about the carbs. Infact I cant even identify the type of carbs I have.

The issues I have are:

*  What are the types of carbs fitted to my engines? I have two engines, and both sets of carbs seem to be identical
*  How do the idling air and fuel circuits work
*  Where can I get spares for the carbs? Amal themselves dont seem to support any of the types of carbs used on the SW and DT engines.
* How are the jets sized? The jet sizes in the literature dont seem to match the jet sizes used by members on their bikes. Are they using the same units of measurement?

Carb types. My engines are both the later small airbox versions, probably 1929 vintage

My carbs all look the same, but dont look like the types Ive seen in pictures in other bikes or any type I can find on line. They also dont carry any codes that mean anything to me. I assumed the carbs were 1928 AMAC Type M carburetters, and I have a manual for this type of carb. However the codes dont match up, and all my carbs are marked on the float chamber tops as being AMALs.

One set of carbs are marked FT 5827 and "6001"and the other carb is marked ?R 5816. The second pair are marked FX2864 and 1/001 and the second carb  FZ5865. These are the carbs featured in the pics below.

All the carbs just have one control, with no air control slide to enrich the mixture for starting.

The air circuits seem similar to those in the Type Ms in my manual. But they dont seem to be working properly. Also there was only one taper screw fitted on all the carbs. The others are just plain ordinary screws.

The jets are identical in design, but differ in size. One carb has 140 main jets, and its partner is a 150. The needle jets dont have any size markings but are marked AMAL. The other pair of carbs have 100 main jets and unmarked needle jets. Douglas said that the jets should be 30% bigger to run on dope, but the jets actually being used by people are hugely bigger (600 ish). Are these sizes all on the same scale?

So to recap I need to know what type of carbs these are. I need to know how the jet sizing works, both main amd needle jets. How do the air and idle circuits work, and why might the carbs not be fueling the engine on idle/start up?

Any information or help gratefully received!

Some pics of one pair of carbs below .....







Offline Buzzie

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Re: DT/SW5 Carbs
« Reply #1 on: 05 Mar 2019 at 22:33 »
My manual

Offline eddie

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Re: DT/SW5 Carbs
« Reply #2 on: 06 Mar 2019 at 06:32 »
Douglas,
                There were at least 2 types of carb fitted to the DT/SW engines. You have the more common type that would have been fitted to the road going machines and some of the speedway engines (it looks very similar to the later type 76 Amals). There was also a sportier carb available for those who were looking for more power at the expense of 'controlability' - these were more like a 'track' carb with no needle in the slide, making them more suited to use with methanol.
  Assuming your carbs are similar to the type 76's - check out the small drillings in the choke block and carb body for the slow running mixture. In the photo (carb2.jpg) there is a hole in the choke block below the main bore - make sure the small drillings into that hole are clear. There is also a small drilling in the carb body from the slow running screw to the main bore - check that that is also clear.
  I would also suggest you ditch the can of 'Easy Start' - that is only bypassing the problem, not curing it - in fact, it may be making it worse!

   Regards,
                   Eddie.

P.S.  I have just noticed that in the photos neither of your carb bodies has a slow running screw fitted - without them, you will have great difficulty getting the engine to start (or run).

Offline Buzzie

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Re: DT/SW5 Carbs
« Reply #3 on: 06 Mar 2019 at 07:23 »
Thanks Eddie, great advice as usual. I'll look at the Type 76 carbs and crawl all over my carbs to check the items you mention.

The carbs Ive pictured arent the ones that are on my engine, and I have robbed some screws off them for the other set. I think there are two adjuster screws in each carb. Are both of these taper ended screws with anti rattle springs fitted? My carbs only had one fitted, all the others were just normal screws. Now I know they are Type 76 like I'll try and source some more of those screws.

Ill ditch the Bradex !

Offline eddie

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Re: DT/SW5 Carbs
« Reply #4 on: 06 Mar 2019 at 08:38 »
Douglas,
              The throttle stop screws are plain screws with a locking nut - the end of the screw is usually turned down to the root dia of the thread for about a 1/4". The slow running screws have the end turned down smaller and have a point for finer adjustment of the idling mixture. These usually have the 'anti rattle' springs.
  If you are only trying to get the engine to run at low speed at the moment, for the time being, I would ignore the main jets - they don't come into effect until 1/2 to full throttle - the slow running settings and the slide cutaway are more important to get good starting and a reliable tickover. Also, pay careful attention to getting both carb slides lifting equally so that both cylinders are doing equal work - this will result in a much cleaner pickup from tickover.

Regards,
               Eddie.