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Keihin FCR39 Breather Hose Plumbing
On the Suzuki DRZ400E the Keihin FCR39 slant’s breather hoses are not plumbed to handle deep water crossings. The result is normally a bike that coughs, splutters and can die midway through a crossing where the water is high enough to block the carb breather hoses. A simple upgrade to the way the carburettor is plumbed will allow the DRZ400E to cross deep water without hesitation or issue.
This image is a DRZ400E which has had the carb breather plumbing upgrade. While the muddy bottom was a bit of work on the engine, it didn't miss a beat and handled the height of the water without issue.
From factory the Keihin FCR39 slant has two T joiners. These are marked by the two arrows in the image above. Each of these T joiners has two hoses routed downward and out by the swingarm. The first step for each T joiner is to (a) leave one of these hoses routed downward (b) reroute the other hose up above the waterline with a longer section of hose. The result is two hoses routed downward and two hoses routed upward.
The next step (the most important step) is to plumb a T joiner into the right-hand breather hose. The result is one breather hose routed downward out by the swingarm and a new section of breather hose routed up above the waterline. You will need to source a 4mm T joiner designed to handle fuel.
From factory the Keihin FCR39 has 5 breather hoses routed downward (plus the overflow hose). To accommodate for the five breather hoses the carburettor has three hose retainers bolted to the bottom of the carburettor with a collective 5 breather hose holes. With this plumbing upgrade, only three breather hoses will be routed downward. To make a tidy job of the breather hose routing start by removing all three hose retainers. Then, use just one of the twin hose retainers and the single hose retainer relocated as shown in the image above.
The breather hose routing when the carb is fitted to the bike looks like this.
The last step is to route and secure the three breather hoses above the waterline. Routing and securing up under the seat (on top of the airbox on the DRZ400) is a good option. I used to route breathers under the seat however I now prefer to route them up by the top triple clamp. The reason for this is if you are pushing the limits and enter a very deep section water, it is possible to get a surge of water that splashes up under the seat. It is at this crucial point you want to keep the engine running and keep momentum up however if the water blocks the three breather hoses, you could find that the bike dies and stops in a deep section of water leading to a drowned bike. For this reason I route the three breather hoses up above the top triple clamp and secure them to the light assembly to avoid this.
Recommended Supplies
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T Joiner
T joiner used to plumb into the breather hose
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Breather hose
The extra breather hose used for the mod to allow for deep water crossing
