Carb Icing

[squirrel_hunter]

I'm planning ahead a bit...

As I intend to spend next week working on my SV, (sorting the misfire among other things), I was wondering if anyone had any ideas to cure the carb icing problem?

Apart from taking them off, cleaning them and checking the hoses and what-not, is there anything anyone can suggest?

Originally Posted by squirrel_hunter

I'm planning ahead a bit...

As I intemd to spend next week working on my SV (sorting the misfire among other things) I was wondering if anyone had any ideas to cure the carb iceing problem?

Apart from taking them off, cleaning them and checking the hose'e and what-not is there anything anyone can suggest?

Dont ride in freezing fog... and if you do, just take it easy. The outside lane of the M25 is not where you want to experience it .

Discussed this last winter with some of the Selkenters. Ironically, it actually happened on the way home that night to top it off. Carb heaters dont work well apparently.

sorry thats probably not very helpful is it??

At least you get carb heaters

[Sid Squid]

The carb heaters do precisely sod all, well, they add a little bit of unnecesary complication and weight, albeit only a small amount of both.

If your bike is affected by carb icing - not all are, and some worse than others - there seem to be only two effective solutions:

1) Fuel additives. Can be dear if you're doing a lot of miles, but probably the most effective way. The most popular variety seems to be Silkolene Pro FST, there are others.

2) Fuel. Shell Optimax has been reported as quite effective, perhaps not as effective as additives, but a whole lot cheaper.

A couple of years ago I made some brackets to lift up the tank about an inch or so, and then ran a duct from the next to the rear exhaust header up into the air filter, this provided slightly warmed air into the carbs and had a very positive effect on carb icing. I didn't go any further with this as I couldn't find a practical way of ducting air under the tank, without having a sizeable tube on the outside of the bike, and tank in my face, but it definately demonstrated that it would work.
Someone else might like to try this, and see if they can come up with a practical duct sizing/routing.

[Scoobs]

I'm lucky that I have not had a problem on my SV, but on my RS125 I could just about manage 20 miles before it packed up. The carb would be encased in thick ice. Fuel additives can help apparently, but as I haven't had the problem (and I do ride all year round) I can't really comment.

[BillyC]

I still don't understand how your fuel can make a difference to carb icing. Classic carb icing anyway. This is probably because I'm not familiar with the construction of the curvey SV's carbs.

I get caught into this argument about once every six months. It's been a while, so here's my money's worth again.

To understand carb icing, you must appreciate two principles:

1 - Air can sustain a given amount of born water. You probably know this as the humidity of air. The warmer the air, the more water it can support as vapour. Air at 15C and 100% humidity, may contain the same amount of water as air at 20C and only 60% humidity.

2 - Charles's Law. This clever chappy noted the relation between the volume of a gas, and its temperature. If you cool a gas, its volume decreases. Alternatively, if you compress a gas, making its volume smaller, its temperature falls.

Carb icing occurs when cool, but moist saturated air is being drawn into the carb.

As the air enters the funnel of the carb, it is compressed and starts to travel faster. Because it is being compressed, Charles's Law says its temperature also falls.

As the temperature falls, the air can no longer support the moisture that it holds, and water droplets condense on the inside of the funnel. The air temperature may fall significantly enough, so that as this water condenses, it instantly freezes.

The ice created forms a build up on the inside of the funnel. This makes the funnel smaller, meaning that the air drawn in is compressed even more - and with that the cycle gets stronger and stronger - colder and colder, more and more ice forms.

This all happens long before fuel has entered the equation.

The only really effective way to combat carb icing, as Emperor Squid has mentioned, is to introduce warm air to the air intake. The temperature drop does not take the air below freezing - therefore no carb icing.

I guess some kind of heat-pipe could be set up between the exhaust heat and the carb funnels, but that might just take the icing elsewhere. The other possible solution would be an alternative airbox, which instead of taking a direct feed of air from the cold air around the fuel tank, takes it from arond the rear exhaust port - but I think this is what Mr Squid suggested anyway.

[TSM]

What about having a waterproof heating element in the airbox, that is temp controlled?

[BillyC]

Originally Posted by TSM

What about having a waterproof heating element in the airbox, that is temp controlled?

Possibly... but remember your airbox is only plastic, and in many cases the air filters are made of paper. It also sits just under a great big tank full of petrol

It would have to get warm enough to heat the air, and still not be a fire-hazzard. You'd have to wire it in with switches and thermostats.

I'd rather just duct a hose that warms its contents over the engine.

Oh dear
Slight misunderstanding here, I am not be pedantic, but there is a fundemental flaw in Billy's explanation-------. When gas is compressed, it heats up, when it expands it cools (aka refrigerator). When fuel & air mix (in the carb), the petrol evaporates, which takes heat. The heat has to come from somewhere & the carb being metal (says he not even knowing if they are on the SV), conducts heat very easily. When the ambient air temperature is warm (say +5 degrees), there's enough heat to go around. When the temperature is cold (say l< 5 degrees), then the evaporation process still requires the same heat, then things start to get dragged down in temperature, the carb gets to a temperature below the freezing point of water & guess what, the water in the air starts to freeze on & probably in the carb--------- There are times I wish I wasn't an engineer, I'd probably earn more money & wouldn't find myself writing sad E mails!!

[jambo]

Sorry Billy, with Paul on this, Gas heats when compressed