LTZ400 Carburetor Hose Routing - Practical Guide
Working on an LTZ400 with the carbs off the bike always makes me want to grab my phone and take pictures first. These things have a bunch of vacuum and vent lines that look identical until you try to start it and it won't run right. Here's what I've learned putting these together over the years. Let me walk through this while you have the carbs in front of you. The LTZ400 uses two Keihin PWK40 carburetors on a 398cc single cylinder. The carb setup on these is straightforward but the ancillary lines around them can confuse people when they're first taking it apart. Starting with the fuel side: you've got the fuel valve on the tank, then a line down to the carb petcocks. Both carbs share a common fuel feed from the tank via a Y-connector. There's a fuel filter somewhere in that line, usually mounted on a bracket near the frame. Return lines go from the float bowl vents back up - these are the small-diameter lines that route to the airbox area. The key thing here is making sure those vent lines aren't kinked. I've seen guys reuse old fuel line that's been sitting compressed and it creates a restriction that makes the bike run lean at high throttle.
Now the intake side. Each carb bolts to its own intake boot that goes into the airbox. The boots are the rubber couplers between the carb throat and the airbox ports. Those boots commonly crack, and when they crack you get unmetered air and a rich running condition that makes no sense because the jetting is fine. Check the inside of those boots when you're doing any carb work. The grommets that hold the boots to the carb bodies also deteriorate over time. The vacuum lines are where people mess up the most. There are two main vacuum ports on these carbs - one for the choke linkage and one for the slide mechanism. The choke vacuum diaphragm connects through a small hose to a port on the intake manifold area. The slide vacuum line goes to the intake tract upstream of the carb slides. If you swap these, the bike will start but the chokes won't work properly and the slides won't respond correctly to engine vacuum. I had a bike once that was running terrible and I spent two hours diagnosing it before I realized the slide vacuum line was on the wrong fitting. The hoses looked the same length so it was easy to mix up. Then there's the crankcase ventilation. On the LTZ400, there's a PCV system that routes blow-by gases from the crankcase. You've got hoses running from the cylinder head area back toward the airbox or intake tract. These are usually the smaller diameter lines compared to the fuel and intake hoses. The CCV hoses can get clogged with oil sludge over time, which causes pressure buildup and eventual oil leaks from seals. I clean these out with carb cleaner and compressed air every time I do a major tune-up.
The carb body itself has several small passages and connections. There's the float bowl drain at the bottom - that's a simple brass screw, but people strip it constantly because they use the wrong tool. There are adjustment screws for the idle mixture and throttle stop. And there are internal passages that connect the pilot circuit, transfer port, and main jet circuit. None of these are visible from the outside but they matter when you're troubleshooting. One thing I always check - make sure the carb mounting nuts and bolts are torqued properly when you reinstall. The airbox-to-carb connection needs to be sealed but you don't want to crack the carb throat by overtightening. Hand tight plus a quarter turn is usually sufficient for those intake boot clamps. If you're looking at a specific hose and can't tell what it is, the best trick I've found is to trace it back to both ends. Start at one connection point and follow the line to where it terminates. The diameter of the hose usually tells you what it is too - fuel lines are typically 5-6mm ID, vacuum lines are smaller around 3-4mm, and the CCV lines vary but are usually in that 4-5mm range.
Get the Full Details
I've found that labeling each hose with a piece of masking tape before disconnecting anything saves a ton of time. Write the source and destination on the tape. It sounds like overkill but when you're working under a quad with limited light and the parts are warm, it makes a real difference. I wish someone had told me that when I was starting out.
