Mapping the hoses on a Big Bear 400 carb setup
Most people who find themselves digging into this are either rebuilding the carb or trying to figure out why their bike runs like garbage at idle. The carbs on these are dual Mikuni-type flat-slide units, and the hose routing isn't intuitive unless you've got the diagram in front of you. There are about eight to ten separate lines depending on your year and whether you have the older or newer style air box. Getting them wrong usually means a vacuum leak, a fuel flow problem, or both. I put together a detailed breakdown because the factory service manual diagrams are decent but they assume you already know which port is which. When I was looking at this for my own 1993 model, the biggest headache was the two small vacuum lines coming off the intake manifold runner plenum. They're easy to mix up because the ports look identical and the rubber tubing is the same color and diameter. The main lines you need to account for are:
Fuel supply from the petcock to the carb inlet. This is straightforward - brass fitting, usually 5/16 inch inner diameter vinyl or rubber hose. Make sure your petcock is on the PRESSURE port if it has that switch. Gravity feed won't give you enough flow for hard acceleration. Return line from the carb overflow port back to the tank or a catch bottle. Don't ignore this one. If you cap it off, fuel has nowhere to go and you'll get venting issues through the float bowl gasket. I've seen plenty of bikes with cracked carb bases from hydrolock during a spillback situation. Intake manifold vacuum lines. These run from the plenum chamber on each side of the intake to various components. One goes to the fuel selector valve diaphragm, another to the crankcase ventilation system. The timing chain tensioner also pulls vacuum from one of these ports on some years. If you're unsure which is which, mark every line with a paper clip or masking tape before you disconnect anything.
Air filter box breather. This is a small hose that equalizes pressure inside the air box. It routes down toward the frame or engine area. Clog this and you'll get restricted airflow that manifests as a lean condition - exactly the opposite of what you'd expect from a breather line issue. Primer pulse or fuel transfer line between the carbs. On the dual-carb setup, there's often a cross-over line that equalizes fuel level between the float bowls. Without this, one carb will run richer or leaner than the other depending on bike lean angle and fuel level. Mismatched carb syncing after a rebuild is almost always traced back to this line being pinched or disconnected. PCV and crankcase evacuation lines. These connect the valve cover area back to the intake manifold. A blocked or kinked PCV line causes positive crankcase pressure, which pushes oil past seals. I had a bike once that was blowing oil out of the rear crankshaft seal at 4000 RPM. Took me three hours to trace it back to a collapsed PCV hose inside the intake manifold passage. The repair was drilling out the passage and replacing the hose - nothing fancy but frustrating to diagnose.
Get the Full Details
Counter-intuitive point: More vacuum isn't always better on the intake manifold lines. The Big Bear 400's carb boot seals are designed for a specific vacuum level. If you've done any porting or changed the intake setup, the vacuum signal changes and your float bowl venting behavior changes with it. The carb may appear to run fine at WOT but lean out badly in the transition range because the venturi vacuum signature shifted. Always check your idle mixture screw count from fully seated before and after any intake work. Another thing beginners miss: The hose from the carb to the fuel filter on the tank side matters more than people think. A too-small ID or a kinked section restricts flow enough to cause hesitation under load without making the bike starve at idle. Use at least 5/16 inch ID fuel-rated vinyl or nylon-coated hose. Never use regular vacuum hose for fuel - it degrades from the inside out and the flakes end up in your jet passages. When assembling the diagram yourself, I found it useful to photograph each hose in place before disconnecting, then take a top-down shot of all the fittings after everything was off. That saved me from guessing which line went where during reassembly. The parts catalog lists these as individual small parts numbers but doesn't show routing, which is why the visual reference helps more than any text description.
One more practical note: the smaller diameter lines (3/16 to 1/4 inch ID) are used for vacuum and breather applications. The larger ones (5/16 to 3/8 inch ID) handle fuel. Mixing these up by getting the wrong hose from the auto parts store is an easy mistake. Label as you remove, not as you install. By the time you're ready to put things back together, you've forgotten which line was which and you're guessing - which is how you end up with vacuum routed to a fuel port and wonder why the bike won't hold idle. Search "Big Bear 400 carb hose diagram PDF" and you'll find several user-uploaded versions. The most reliable ones come from Yamaha service documentation groups. Look for the ones with the 1991-1999 part number references. Avoid the generic ATV forum images that are just someone's phone photo of a faded printout - those often miss one of the smaller breather lines.
