Hi Paul,
I have had similar. But mine was the carb actually jamming open. Turned the ignition off, felt the carb and it was icy cold, held a warm hand on it for a while then heard the slide clunk back down.
So, yes, they can suffer from icing. Does not really help as I do not have a solution.
Do you have a large insulating washer between the head and the carb?
Hopefully someone else will come up with an actual solution.

Update, A bit of research has come up with:-

Silkolene Pro FST

Multi-functional fuel treatment. Protects against cold start wear, corrosion, carburettor icing, deposit build up problems, stalling and rough running at low engine revs.

No idea if it is snake oil ...

Tony

... normally occurs when there's high humidity and air temperature is above freezing. So if your weather is like ours here - dry but around zero - I'd not think it was that.

How old is the petrol, as summer blends are different to winter ones as I Understand it.

I used that Silkolene product many years ago when I was running bikes all year round and felt it made a difference. But probably just wishful thinking.

Some people wrap the carb with old socks or similar and claim it helps prevent icing.

My Triton stopped icing the carb up when I changed from a single 389 to twin 928s.  No idea why that fixed it.

Hi Michael,

It is the high speed air going through the carbs that causes the drop in temperature. By going to twin carbs, you have halved the amount of air going through.

Tony

My 99DL has stopped from icing several times. It allways re-starts after a while. Its not uncommon on bikes without a carb heater , Old VW bettles could do it too. perhaps substituting an alloy spacer for the phenolic one during the winter months could  prevent it. Perhaps the rear enclosure deflects the engine heat away from the carb? Possibly after starting and warming up we should wait a while to allow heat to transfer from engine to carb. An electric element wrapped round the carb body would be a good fix ,an old 12v  heated bar grip ?. I have an old  12v campers Hair drier ,arrange for it to blow warm air into the Air filter !!, My bike once had some fibreglass engine enclosures also fitted ,the previous owner rode it all year all over europe ,perhaps the bike ran a lot warmer from it?. Enclosure was called "Fairshields " probably by Avon ,never seen another.

Could it be due to a blocked tank vent hole  Similar symptoms!
Laurence

I have an original Ewarts single plunger tap that incorporates a reserve . The reserve holds very few miles use . It also does not seal well between main and reserve . If you have a very economical machine and ride at 40 mph it's just possible to drift along on the leakage and not realise until you are completely dry! It only works if your bike is capeable of repeating the feat that the official road test machine did, over 100 mpg! . 

My 99 was very prone to carb icing in winter. Generally just stopping the engine for 3 or 4 minutes to let the engine heat get to the carb would melt the ice and all would be well. What can happen though especially on a fast dual carriageway is that the ice build up jams the throttle open so that as you approach the end of the carriageway and see the roundabout looming the throttle won't shut.
The other effect of carb icing is that the partially blocked carb runs rich and you can get black smoke and awful petrol consumption.
Speaking from experience here.

• You can measure how long it takes a litre to run through the carburettor.
• You can see how it goes on a warmer dry day.

The vent in the tank cap can be tested too. If it has the visible little hole in the top surface, blow through it into the tank.

 Had a similar problem on my model 50, in good weather though.  Engine would stop after 5 to 10 miles.  Found the problem when I was draining the petrol tank, good flow of fuel then a dribble. Turned out both petrol tap filters were clogged with a grey paste. Replaced both taps and removed the fuel diamond which had been in the tank when I bought it.

Sounds very much like fuel starvation, easily tested next time it happens crack the fuel cap, if the bike picks then you have the answer.  

Funny how sometimes we live in a different world to our mates! Never had carb icing. But I have had the reverse on a Reliant.
As built (in the 70s) the carb was mounted direct to the (water) heated manifold. Fag shop syndrome, stop at fag shop, return to car, restart was very i'fy' and driving became hit and miss for a couple miles. Boiled carb.
Answer-fit a carb insulation washer. When we told the factory, they agreed they had a 'sometimes' problem but not enough to spend 30P on parts to put it right in manufacture. Not surprising the British motor industry failed.

I've found that the NOC shop has a range of reasonably priced taps (that include filters) - order Sunday evening and get before the next weekend for a very low postage charge.
Could the looping round of the fuel pipe near the warm engine cause an issue?

Don't let the petrol hose touch the head or barrel!

I remove petrol tap gauze and fit an inline filter.  The gauze is a built-in breakdown.

Hi, Paul.

To answer your query the filter is part of the  tap and is inside the tank so you need to remove the tap to ensure it's clear. There is another filter (or should be) inside the union the petrol pipe connects to on top of the carb.

As mentioned, I think it would be best to route the pipe away from the hot engine. Regarding having the pipe routed so it has an upward trajectory to the carb, this doesn't matter as long as the reservoir (i.e. the tank) is above the carb.

I would suggest using a Dowty washer to seal the tap to tank since this is a little pliable so allows you to have the tap facing the way you want it to be without being over tight, as you would need with a fibre washer.

HTH.

The Tap looks to be the very rare  and desirable orriginal Ewarts .  the fuel pipe run is pretty much as built except you have a very usefull extra  90 degree elbow that allows an even better run than the standard arrangement for a DL that is usually pinched by the sidepanel .I shall fit an elbow just like yours. Stick with it!  I have just replaced my orriginal (64 years old plastic pipe ) as it was very hard and ridgid,  and i thought it may shatter sometime.  It been a real bother trying to avoid it getting pinched by the sidepanel. the tap may need a new cork fitted , no bother i have found . No one  except ME ! seem to have any new extended nose  plungers of the correct type . But they all think they do ! Fools all. The corks are availiable but need to be a tight fit if they are to last and seal . I can detail the process if you get in trouble . Do not lose or damage the plunger retaining screw .You wont get another and they are not simple .Corks will last for years if fitted right. They get worn from the rust particles that escape the filter.Wash out the Tank. Never had vaporization trouble but had freezing carb.

The hot water may help for a while and bar soap will lube and fill imperfections in the cork. .the knob can be punched off and a new cork fitted knob re riveted . Cork  needs to be  bigger than the hole. be carefull to take photos before as there are two ways to assemble . some just slit cork end to end and fit.,good cork has little or no  imperfections.you may have to buy corks from different sources to find ones that fit. rub round with fine sandpaper if much too big. Needs to be very tight and will bed in with use.  Fit an air filter as stones get thrown forward by back wheel. No oil leaks ! very good. !!

One thing... And I know there's some quirks on the bikes. Is the thread for the tap into the tank typically clockwise doing it up? Don't need to be breaking the tank! Thx

Don't think I've ever seen left hand threads in the fuel system.

It's all righty tighty, lefty loosey !

I tip the tank contents out over an old kitchen bowl and with tank still upside down wipe inside the filler cap area with kitchen roll to get the grit .after standing the fuel goes back in as water and grit settle in the bottom .  Note ! I did say OLD kitchen bowl !!

Nothing to do with carb icing, if it was you would feel a very slack throttle as the cable no longer pulled the slide when it happened.
Petrol pipe 'through' the manifold is causing petrol vaporising it appears to run out! by the time you slow it will probable clear but it has to refill the float chamber, so takes a moment or two. Ie you stop and park up-all is now well.

You can have carb icing without a stuck throttle slide.

I agree Michael, suffered icing on a number of occasions ,no slide issues at all . frost forms around the air bleeds on the needle jet stopping the flow of fuel.