Carb Icing Explained

[Tooty]

Just want to put the record straight on carb icing as there a few myths floating about.

Carb icing can occur at air temperatures between 0 decress and 23 degrees Celsius.

It cannot occur at temperatures below freezing.

Carb icing is worse in high humidity conditions i.e. Fog.

Carb icing is actually ice crystals forming in the low pressure areas of the carb, i.e. the venturi. Those of you who paid attention at school will know that when you put a gas under pressure the temperature will increase. Conversely when you drop the pressure of a gas the temperature will decrease.

This is whats happening in your carb. At low cruise throttle openings or when the engine is on the overrun, the pressure in the venturi drops. The drop in pressure will result in a drop in temperature. If the drop in temperature low enough and is sustained for long enough, ice cystals will form. These are what makes the bike run rough and in extreme cases will stop the engine.

Petrol additives won't help as these just stop the petrol freezing. The ice cystals don't come from the petrol, they come from the air and there's nothing you can do about that!

Piston engined aircraft suffer from this problem a lot and they get around it by directing warm air onto the carb. Other methods are electrically heated jackets or heated elements using warm radiator coolant.

The design of the carb and inlet tract are the main contibution to carb icing which is why certain carbs and air boxes are more prone than others. For info, Fuel injected engines don't suffer from carb icing at all, which is another good reason for buying a fual injected bike.

Tooty

[Gas Axe]

Good expanation mate,I have noticed my carb probs occur after opening up gently after Being on the overrun going into a corner.I think it could also account for slight backfireing on the overrun,which I have found impossible to tune out with the pilot screw.I couldnt understand originally why the backfireing only occured as the revs were well down e.g. after shutting of for 5or6 seconds ( probably enough time for manifold pressure to have remained low enough to form ice ).
I have noticed a spring loaded valve in the butterfly on car carbs of the seventies,this would relieve the low pressure on shut off,maybe that was an anti iceing valve.It wouldn't work on a c.v.,you would lose a lot of engine brakeing.
Do you suppose isopropyl alcohol is a bit of a plecebo then,or maybe it evaporates more redily without robbing it's surroundings of as much heat as petrol.
Any idea which part of the carb is affected by the ice crystals,is it the pilot
jet?
Keep thinking
Cheers
Gas Axe

[Walenut]

It could be the air pilot jet which would then cause it to run rich, like running with the choke on, interesting stuff.

We use the venturi effect to draw in other fluids etc as it creates low pressure, remember that one Baza.

My VTR has a small circuit from the water jacket around the carbs to prevent icing.

[Roddy_the_Dog]

You could argue this is a design fault,I regularly went to breakdowns on a Bedford van as an apprentice 30 years ago,by the time I'd get to it eveything was fine,this went on for weeks till I found a crack in a convoluted tin foil hose that directed heat to the carb.

So carb icing is nothing new.

[HOG maniac]

Winter Idle boost and optimax stops carb Icing. The Ice may come from the air but the flow of petrol will remove it again if it has the correct additives. The Ice on your windscreen comes from the air but deicer removes it.

[Chiefy]

Originally Posted by HOG maniac

The Ice on your windscreen comes from the air but deicer removes it.

This is a different set of circumstances to carb icing.

[HOG maniac]

Oh ok. I used to get it bad but optimax and Idle boost from nov to march sorted my issue

[Chiefy]

Originally Posted by HOG maniac

Oh ok. I used to get it bad but optimax and Idle boost from nov to march sorted my issue

Fuel conditioner has almost sorted the problem for me. But this treating the symtoms rather than a cure.

A lot of Jap bikes have carb heaters of some form or other. Which would have been a cracking idea if fitted by HD.

But it is a Pressure / temperature problem rather than just a temperature issue.

B*llsh*t over, I guess if you can get the bike to run better with Optimax and Idle boost then thats the answer you need.

[Ukkid76]

Replacing your stock carb to a Mikuni HSR42 will stop carb icing without having to but a petrol additive in your tank weather you use 95 or 98 octane fuel, plus you get much better engine response and tuning options, all round a better option I would say!!

Ukkid 76

[Chiefy]

Originally Posted by Ukkid76

Replacing your stock carb to a Mikuni HSR42 will stop carb icing without having to but a petrol additive in your tank weather you use 95 or 98 octane fuel, plus you get much better engine response and tuning options, all round a better option I would say!!

Ukkid 76

A perfect solution.

If you have the money