Generator carburettors are prone to becoming gummed up with contaminants from old fuel, a deposit called varnish. Without the addition of a fuel preservative, this can happen in as short a period as one month if the recreational vehicle, or RV, is unused. When this occurs, it will be necessary to remove and clean the carburettor as part of flushing the entire fuel system. Onan generator carburettors are typically of a primitive design, and their removal is straightforward so long as the layout of the RV allows easy access.
Use an adjustable wrench to disconnect the negative cable of the battery which serves to start the generator. This will typically be the coach battery. This is particularly important if your generator has a dedicated low-pressure fuel pump fitted in-line.
Determine the location of the carburettor on your Onan generator. Use your owner's manual or follow the fuel line.
If there are any control wires attached to the carburettor, use a tiny spot of different coloured nail polish to mark each spade connector and its terminal and then carefully slide the spade connectors off the terminals. Put the wires where they cannot be harmed. Onan carburettors do not typically have wiring to them.
Use an adjustable wrench to free the nut holding the fuel line to the inlet port of the carburettor. Gently draw the fuel line away from the inlet, and cover the end with masking tape to prevent the ingress of contaminants.
Use a properly sized socket to remove the bolts holding the carburettor to the body of the generator's engine. There will typically be two bolts, but some Onan models have more. Put the bolts where they won't be lost.
Draw the carburettor away from its location, trying not to damage the gasket. Use a rag dipped in a solvent such as acetone to clean the face of the engine where the carburettor was sited, and then bunch up a clean piece of rag and insert it slightly into the hole in the engine to keep contaminants from entering.
Use a digital camera to take photos of the carburettor before removing it from the Onan generator. These will make valuable reference guides when reinstalling the unit. Older gaskets may fracture when the carburettor is removed. Be sure to check for this, and use new gaskets if necessary.