Where Do These Hoses Go?
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This is the standard vacuum operated petcock on the classic Suzuki GS motorcycles.

Vacuum petcock

The smaller hose on top is the vacuum hose. It connects to the #2 carb if your bike is equipped with Mikuni BS series CV carbs.
The larger hose on the bottom connects to the fuel inlet between the #2 and #3 carbs.

The petcock has no "OFF" position. This is a vacuum operated petcock. If the engine is not running there is no vacuum being supplied to the petcock to open the internal diaphram and allow fuel to flow.  The petcock above is shown in the "ON" position. This is the normal running position. Move the circle pointer to "RES" to activate the 'Reserve' setting. Move the circle pointer to the "PRI" position to 'Prime' the carburetors in the event the fuel level in the float bowls is low due to evaporation or you have just re-installed your carbs after servicing them.

For the other ends of the hoses, look at the following pictures.

CV Carb hoses

The above is a view from the back side of the carbs, the airbox side. Below is a detailed view of the hose connections from the front side of the carbs.

Detailed CV Carb Connections

Here's another shot of the hoses labeled for use.

CV Carb Hoses Labeled

Here are a few words from Mr. bwringer about hose sizes:

The fuel hose on an 850 is 8mm. 5/16" works quite well, since these match up almost perfectly when you convert (5/16" = 7.94mm).

The vacuum fitting is 6mm. Some 1/4"     hose is too loose (1/4" = 6.35mm), but some brands are a little tighter and work fine.
(NOTE: I found some 7/32" hose at the local Ace Hardware store that worked perfectly.)

I believe the breather hose is either 10mm or 12mm. 3/8   " hose converts to 9.5mm, so it'll work OK on a 10mm fitting.

If it's 12mm, most 1/2" hose will be a bit too big, since 1/2"=12.7mm. Again, you'll need to experiment a bit if you replace this with American sizes. (NOTE: You may be able to find 7/16" hose at your local hardware store.)

And finally, the vent hoses are 5mm. 3/16" hose converts to 4.7mm, so that's close enough. It'll be a wee bit tight, but not really noticeable or a problem.

Ordering the OEM stuff works well and you usually get more than you need.

On those bikes with fuel gauges, you will have a fuel gauge send unit on the bottom of the tank. In the picture below you see an overflow hose, in case the send unit ever leaks, and a wire with two connectors, which connect to the fuel gauge connections labeled in the picture above. The drain hose runs down in front of your battery and hangs down at the rear of the engine.

Fuel Gauge Send Unit

There is also a drain hose on the bottom of the airbox as well as a breather hose on the top that connects to the breather on the top of the engine's valve cover.

Breather Hose

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