Some insight regarding that carburetor that ices up
Question:
I’m writing in response to the August question about the 1973 VW Karmann Ghia that has the carb icing problem. (The reader said he’d had the VW’s engine rebuilt and at first it wouldn’t idle. After some work, he said the car would idle, although it seemed high, and “the stand the carb sits on ices up... This is in the engine compartment of an air-cooled VW. Not exactly a cool environment. Actual ice, on the stand!” He said that during the rebuild he had sandblasted all of the engine tin himself “including the manifold with the carburetor stand and heat riser tubes (all one piece).” He went on to say there were no obstructions in the tubes.)
The reader stated that the intake and heat riser had been sandblasted and were open (one-piece manifold). I would like to think that with his completion of the Ghia that the exhaust was also replaced? That would be the first place that I would look.
The stock exhaust on all OEM VW air-cooled engines with a single carb have a built-in UN-balanced exhaust. This causes a difference in pressure from one side to the other allowing HOT exhaust gases to flow through the heat riser and heat the intake in order to prevent icing.
A “high-performance exhaust” does not take this into consideration as most of these are used on dual carb set-up engines. When converting to the dual carb set-up this is not a problem as the intake runners are much shorter and there is less air flow through each carburetor, which means less cooling effect.
The icing issue also happens when the heat riser tube gets plugged up or restricted and the exhaust pressure differential is not enough to flow through the heat riser tube.
Answer:
Thank you for taking the time to share your experience and expertise with fellow Auto Restorer readers. I did not realize that installation of an aftermarket performance-type exhaust could be a factor with carburetor icing. This may be very helpful information for Cliff Poirier, who wrote in about the problem initially and any others in his situation.