Carb freezing

For what it's worth,  many of the last Japanese bikes with carbs had heaters fitted as close to the pilot circuit as possible.  These  were normally between 15 and 25 watts and screwed into a 6 or 7mm hole (cant remember exactly ) on the underside.

A bimetallic switch turned them on when the air temperature was less than 4 degrees Celsius. It was found that icing would not happen above. 

i used to have a 750 Commando with a single Mikuni, it used to suffer dreadful carb icing.

It used to cut out and the only cure was to sit on the bike for 10+ seconds and it would then start up again.

never did sort it out, but these days I tend not to ride in freezing conditions .

sam

On my Commando one cold night I had done plenty of miles with cheap supermarket petrol in it.. I refueled with expensive shell petrol and within ten miles the carbs iced up. There was another Commando with me who had exactly the same experience. Maybe some makes of petrol need more heat from the carb body to vaporise the fuel. 

... that you get carb icing. It's due to something called the latent heat of vaporization. Essentially, changing state from liquid to vapour requires heat which comes from the carb body cooling it down. If there's a lot of moisture in the air and the temperatures are low (but not freezing or below) the drop in temperature causes the water vapour to condense out and freeze.

In theory, petrol shouldn't actually vaporize in the carb but should be turned into a fine mist of tiny droplects.

A few of my mates with Ducati Monsters suffer from carb icing. (Not me.. Mine's injection.)

It was a know fault in the days of carbs, so Ducati sometimes fitted either electric or oil fed carb warmers.

Those with no warmers swear by Pro-FST, which is a petrol additive, but you have to put it in every fill up in anticipation of icing conditions... It needn't be that cold actually, it's all down to relative humidity. 

You're a better man than most if you know for absolute certain that the petrol you buy is ethanol free,. even if like me you only buy the premium, super or plus grades.

However the problem probably isn't ethanol as you say, but fuel composition has changed radically over the years, even since it went lead free.

I'd like to think that the powers that be are evolving the fuel into something a little friendlier for our children than it was "back in the day". It's just a shame that old bikes don't like it!

Injection systems don't suffer from icing as they don't have a venturi. Certain carbs suffer more than others and I think it might be due to the shape of the venturi... This is where a designed pressure drop occurs, and what causes the fuel to atomise. The pressure drop is also what draws the water out of suspension in the air. You can see this graphically on the trailing edges of a high performance jet when it performs high G manoeuvres in cool moist air... The water snaps out of suspension in the ultra low pressures in the plane's wake.

I think you might be on a hiding to nothing by trying to rig a carb warming device, besides which the "icing" season is hopefully nearly over till the autumn.

I reckon it would be well worth trying a few doses of Pro-FST for the next month or so and see how it goes... No modifications to the bike required.

I wonder if there is a connection between icing and having a single carb?  The single Mikuni that Sam refers to is being forced to atomise twice as much fuel as each of the original Amals.

I had twin Amals when mine iced up.