Have you ever experienced carburettor icing on a buggy? Driving along the motorway and your buggy seems to lack power, so to compensate you push the throttle down more… and burn lots more petrol… eventually you stop to have a look and see condensation on the carb. You wait a couple of minutes and off you go again with no problems until it repeats itself after 15 miles or so.
You will find that the symptoms are worse on some days than others. The answer lies in the relative humidity of the air, and the air temperature. We all know that the function of a carb is to turn petrol into vapour and mix it with the air. In vaporising, the fuel draws heat from the inlet tract, notably the carb and the manifold neck immediately beneath it, and in doing so cools the metal parts to below the freezing point of water. The expansion of air after being drawn through the restriction of the venturi, combined with the chill of cold fast-moving air, cools the carb sufficiently to ice the fuel, which then gradually closes the intake to air and sucks more fuel to equalise the manifold depression. There’s always some moisture in the ambient air, and on days of high humidity there’s more. This moisture condenses out on to the cold walls of the inlet manifold and because of their temperature can turn to ice. Although that fuel may be burning, it won’t be doing so efficiently when there’s ice on and inside the inlet manifold. The petrol tends to de-vapourise back into large droplets, which trickle into the intake path and cause rough running, flooding and stalling so well known to us air-cooled VW owners.
In an attempt to combat this problem, Volkswagen designed a warm-up system whereby hot exhaust gases could flow through the heat riser pipes, to transfer heat to the carb base, to prevent carb icing.
With some after-market single carb set-ups, these heat riser pipes are non existent, made mainly for hot & sunny climes! With dual carbs, icing is not so much of a problem due to the short length of the manifold. On my red GP buggy (single Dellorto carb) I find the problem normally occurs when the air temp is below 10 degrees. To overcome this, I have a setup that looks naff but works! Basically warm air from one heater outlet is redirected to carb filter unit.
Suggestions as to how to reduce carb icing on a buggy:
1. Fit dual carbs rather than single.
2. Direct warm air into the carb via filter or carb base by additional pipework.
3. Fit an IMDU (inlet manifold de-icer unit) this is a warming jacket you wrap around the base of carb, they are normally for sale at the Slough Swapmeet in November -
4. Ensure heat riser pipes aren’t blocked with solidified carbon from exhaust.
