Weber IDF/IDA Phenolic Insulator Kit
GansMann Industries Exclusive
Fits: All IDF and IDA style carburetors
These are a quick fix for carburetors being subject to "heat soak," where fuel is boiling in the float bowl. Modern fuel begins to boil around 160 degrees Fahrenheit at sea level, but can vary with ethanol content. I've found that the issue began on a Type 4 bus motor when the carburetors reached around 150 degrees F.
Please verify that your float settings and fuel pressure are to spec before ordering spacer, If either of those are the underlying cause, it can be a waste of money to put insulating spacers on.
If you're sure you have the carbs set right but you get the percolator drip on shut down, feel the carbs 5-10 minutes after shutting the motor off. If they are very warm to the touch you probably have heat soak issues.