Carb Icing

A carb works (roughly) by sucking petrol out of a chamber and mixing it with air. To achieve this 'suck' the air has to be at a slight vacuum. This is brought about by the carb having a narrowing of the port through which the air passes, this is calles the venturi. However Boyles law tells us that with that slight vacuum you'll also get a temperature drop. Normally this isn't a problem until the inlet air is at about

The only cure is to make sure the inlet air is warm enough. This is done by trnking from the exhaust manifold and some sort of switch to direct the hot air when it's needes. The switch can either be manual or automatic.

John

Good explaination ... thanks. I've been running an awkward setup on my citroen for 3 winters now (all the pre-heat system has been removed then a k and n air filter fitted, plus both primary and secondary jets have been increased in size by 8 percent). If i start up with the ambient temperature below 8 degrees, within 4 miles of 1/3 throttle driving the car will start hesitating due to weak mixture. Obviously giving it a boot full opens up the second throttle butterfly and all is fine. (Its a twin solex carb).

Best cure in my case is to switch off and wait in a layby for 5 minutes for the heat to rise into the carb.

One other thing .... the carb on mine is electrically heated ... i wonder if switching to "run" 5 minutes prior to starting would help??? Carb icing seems to go after 20 mins of running reagrdless, once the engine bay is fully warmed.

H>

Erm, have I got this right - You've got K&N filter and rejetted carb, doubtless for performance reasons, but you have to stop for 5 minutes in a layby to let your carb thaw? Sounds like one step forward, and a whole heap of steps backwards to me. Surely you could lash something up for winter use - bit of ducting over the K&N, drawining in air from near the exhaust manifild, maybe?