1923 TS cant get slow idle

[Kidamabush]

Hi, Les here. I have recently acquired a 1923 2 3/4 TS. It is fitted with a Douglas automatic Carb. I have no problem in getting the bike started and running but cannot get it to idle at any reasonable low speed. It always seems to race. This is regardless of Air lever or throttle position. Any ideas

[cardan]

Hi Les,

Is this the carburettor? https://www.douglasmotorcycles.net/index.php?topic=7819

Vintage bikes often idle poorly, in part because of the rudimentary pilot jets fitted to period carbs. If you can't get a low idle, a reasonable plan is that the engine stalls when both levers are fully closed. Once you're sure that the cables let the throttle and air close fully, check for air leaks. A leak anywhere between the carb and the inlet valve can cause the engine to race, even air leaking in around worn inlet valve guides.

A good way to search for leaks is with a spray bottle of alcohol (or even water if you don't have alcohol): squirt the various joints while the engine is "idling" (racing), and listen for changes as the leaks are temporarily sealed.

Leon

[Kidamabush]

Yes, that is the carb. I know both the throttle and air close fully so I will do some checks for other air leaks. Thanks for your guidance.

[Chris]

Hi Les
I have always referred to this particular carburettor as the rotary barrel carb. It is the normal fitting for the CW model. Many years ago when I purchased my CW.it was exhibiting exactly the same symptoms as you describe. The previous owner had been unable to cure the problem. He put it in the hands of the late GD Brown of Godalming for a service and MOT. Mr brown gave it a pass for the MOT but suggested he replace the carburettor. He then wrote to the late Frank Dolman who assured him that to change the carburettor would require the manifold to be changed and said that although the rotary barrel carb was not exactly brilliant it should work reasonably well. The previous owner then gave up and covered the bike with an old carpet in his garage where it stayed for several years before he sold it to me. I carried out a comprehensive restoration including a rebuild of the engine and to my dismay having completed the restoration I still had the same problem. Being absolutely certain that it was the carb. causing the problem. I dismantled it completely studying each component and trying to understand how it worked. Looking at the bore of the rotary barrel it suddenly dawned on me that the sculptured shape of the bore was the wrong way up. I then realised that it could be assembled 180 degrees out. Turning it upside down and securing with the two screws made sense. Put back on the bike it started easily and ticked over like a grandfather clock. This was many years ago and the bike is still running well and has been used with a sidecar attached in Belgium, France, the Netherlands and in many events in the UK including awards for best three wheeler in The Banbury Run. Obviously I cannot guarantee that this is the problem with your machine but it immediately brought the history of my machine to the fore.  Chris Wright.

[Kidamabush]

Hi Chris,
I had a good look at my carby over the weekend and when I pulled it apart it did look like the barrel was upside down as the sculpted part of the barrel was upright and the open portion at the base.  It think at some stage someone has given it a clean and put it back together the wrong way. The previous owner hadn't really run the bike at all, just knew that it fired. As there is only a single locking screw to turn this 180 degrees I gave it a go. While the result is greatly improved I think that there is so much wear with the barrel that some of the effect is negated. Thanks for your help with this idea, as while logical would be difficult to find. More work to go but its much better. I am going to try to find another carby and see if that makes any improvement. Possibly even another of the carb types fitted. Thanks for your help.
Les

[Hutch]

Hi Les,
I have a two of the Douglas Updraft Rotary Barrel Carburettors and both are worn and would probably leak air between the shaft for the barrel and the bush and also  in other locations – just like yours. Guess the bush could be machined out and a sleeve pressed in and also the shaft cleaned up easily enough. There is also a shim between the barrel and the flange and I guess the thickness of this could be critical in eliminating leakage around the sides of the barrel as well?

An alternate carby (Edit:- That would suit your updraft inlet manifold) that was offered by Douglas for the CW and possibly TS was the Amac T30 VD (….I presume VD is for vertical-draft as opposed to side-draft ...no not the other! 😊 )

First picture is from “List of Assembled parts for Douglas Motorcycles Models TS and CW” from 1925. The second picture is of the “Amac T30 VD Carburettor from Carburettor Spare Parts for Amal Binks Amac B&B Carburettors 1923-1030 Inclusive” which can be found in the Barnstormers website here;

 https://www.barnstormers.co.nz/259/vintage-carburetters-pt-1/

One of my Douglas barrel carburettors has an interesting feature on the air slide – I will get some pictures of it and post when I get a chance.

Cheers

Hutch

[cardan]

Some things to keep in mind:

Idling performance is not very important on a TS? Without a clutch, you'd usually paddle off and stop with the exhaust valve lifted? Best to check how the bike runs along the flat and pulls up hill before concluding that your carb needs changing.

If the bike really is a 1923 TS, I think neither the Douglas Automatic nor the AMAC T30VD would have been fitted originally, but instead one of the horizontal AMACs. It might be a good time to check what bike you've got and what carb was originally fitted. The Douglas carb came originally with the Douglas double lever.

You can swap carbs until the cows come home, but if your valve guides are worn or your manifold leaks the bike will never idle nicely.

Cheers

Leon

[eddie]

I have not had any dealings with the Douglas carb, but engines with 'normal' slide carburettors often wont tick over slowly if the carb body has become worn. Nothing makes itself obvious when inspecting the carb with the engine stopped - but with the engine running, the vacuum in the manifold pulls the slide against the worn portion of the body and wont allow it to drop fully. On hearing about this, quite a few club members have looked at me as if I was going mad - that is, until I offer to re-machine the body and slide - then they find out what a difference it makes!

  Regards,
               Eddie.

[graeme]

This is absolutely true Eddie

I had the same issue with my Triumph T160 Trident when I first got it in 1983 (wow, so long ago now  ), the idling was up and down like a yoyo. A friend who is a clever machinist offered to clean up the bores of the carbs and sleeve the slides to suit, and the difference was astounding! Dead even idling, and it has been like that ever since, with a further 60,000 miles put on them

Cheers, Graeme

[Kidamabush]

An update on progress. To test whether the carb wear was the biggest factor I took the option of making a simple right angle manifold adaptor so I could fit a more modern side draft carb (only a couple of hours of machining and fitting to make flanges and using a 1" copper tube elbow).I had an old Jikov from a Jawa that is similar in bore dimension and jetting so this was fitted.  I am glad I did this as the result was incredible. Start and run smooth on 1st kick, good pulling through revs. While this is a usable option I still would like to return to original so I may start to explore more on getting the barrel sleeved/refurbished. Thanks to John S for sending me some info.