If your quad is starting to sound a bit like a sewing machine or it's becoming a pain to start when it's cold, it's probably time for a 400ex valve adjustment. The Honda 400ex is one of the most reliable machines ever built, but it's an air-cooled engine, and those valves tend to move around a bit as things heat up and wear down over time. It's a standard maintenance task that a lot of riders ignore until the bike starts running poorly, but honestly, it's something you can knock out in your garage on a Saturday morning without much stress.
Ignoring your valves isn't just about the noise. If they get too tight, they won't close all the way, which means they can't dissipate heat into the cylinder head. That leads to burnt valves, and suddenly a simple adjustment turns into a full top-end rebuild. On the flip side, if they're too loose, you're losing lift and duration, which just kills your power. So, let's talk about how to get this done right so you can get back to the trails.
Getting the Right Tools Together
Before you start tearing things apart, you'll want to make sure you have everything on your workbench. There's nothing worse than having your quad half-disassembled and realizing you don't have the right feeler gauge. You'll need a basic socket set, specifically a 10mm for the adjuster nuts and whatever sizes fit your plastics and fuel tank. You'll also need a large flathead screwdriver or a coin to remove the timing inspection plugs on the side of the engine.
The most important thing, though, is a set of feeler gauges. If you can get the "offset" or bent feeler gauges, your life will be a lot easier. The space inside the 400ex head is pretty tight, and trying to shove a straight gauge in there is frustrating. For a stock 400ex valve adjustment, you're looking for .004 inches on the intake (the ones near the carb) and .005 inches on the exhaust (the ones near the header).
Prepping the Machine
First off, make sure the engine is stone cold. Do not try to do this right after a ride. Metals expand when they're hot, and the clearances you're measuring are so small that heat will completely throw off your readings. Let it sit overnight if you can.
You'll need to get the seat and the gas tank off to really see what you're doing. Some people try to sneak in there by just propping the tank up, but for the ten minutes it takes to pull the tank, it's worth the headache you save. Once the tank is out of the way, you'll see the four valve inspection caps. They look like big hex-head bolts on the corners of the cylinder head. Go ahead and pull those off, but be careful with the O-rings. If they're flattened or cracked, you might want to replace them so you don't end up with an annoying oil leak later.
Finding Top Dead Center (TDC)
This is the part where most people get tripped up. You need to make sure the engine is at Top Dead Center on the compression stroke. If you're on the wrong stroke, your valves will be under tension, and your adjustment will be totally wrong.
Pop the two plugs off the left side of the engine case (the stator cover). Use a socket on the center nut to rotate the engine counter-clockwise. Watch through the smaller inspection hole for a little "T" mark. You'll see an "F" first (that's for firing/ignition timing), but you want the "T."
Now, here's the trick: just because you see the "T" doesn't mean you're on the compression stroke. Give the rocker arms a little wiggle with your fingers. If they move just a tiny bit, you're good. If they're rock solid and won't budge, you're likely on the exhaust stroke. If that's the case, rotate the engine one more full 360-degree turn back to the "T" mark. Now both the intake and exhaust rockers should have a tiny bit of play.
Checking the Clearances
Slide your feeler gauge between the top of the valve stem and the adjustment screw. For the intake side, grab that .004 gauge. It should slide in with a little bit of resistance—kind of like a magnet pulling on a fridge, or like sliding a business card into a tight slot. If it slides in and out with no resistance at all, they're too loose. If you can't get the gauge in there at all, they're definitely too tight.
Repeat the process on the exhaust side with the .005 gauge. The exhaust valves usually run a bit hotter, which is why they need that extra thousandth of an inch of clearance. It's pretty common to find the exhaust valves a little tighter than the intakes because they take more abuse from the heat.
Making the Adjustment
If things aren't perfectly in spec, don't sweat it. Loosen the 10mm lock nut on the rocker arm. Once that's loose, use a flathead screwdriver to turn the adjuster screw. Small movements make a big difference here. Turn the screw a tiny bit, then hold it steady with the screwdriver while you snug the lock nut back down.
Here's the annoying part: sometimes when you tighten the lock nut, it actually turns the adjuster screw just enough to change your setting. It takes a little practice to get the "hold" right. After you tighten the nut, always re-check the gap with your feeler gauge. If it's still perfect, you're golden. If not, loosen it up and try again. It's worth being a perfectionist here.
Putting Everything Back Together
Once all four valves are set and you've double-checked them, it's time to button everything up. Check those O-rings on the inspection caps one last time. If they look dry, a little dab of clean engine oil can help them seat better. Don't over-tighten these caps; they don't need a ton of torque to stay sealed, and you really don't want to strip the threads in your cylinder head.
Put your timing plugs back in on the side of the case, throw the tank and seat back on, and you're ready to go. When you fire it up, you should still hear a very slight "ticking" sound. In the world of air-cooled Hondas, a "tappy valve is a happy valve." If it's dead silent, they might be too tight. If it sounds like a hammer hitting a tin can, they're too loose.
Why Regular Maintenance Matters
Doing a 400ex valve adjustment every 20-30 hours of riding might seem like overkill, but it really keeps the engine crisp. You'll notice it starts easier, idles smoother, and has a bit more snap when you crack the throttle. These engines are bulletproof, but they do require this bit of "manual" love since they don't have hydraulic lifters like a car.
If you've never done it before, the first time will probably take you an hour or two while you figure out the "feel" of the gauges. But once you've done it a couple of times, you'll be able to fly through it. It's one of those skills that every 400ex owner should have. Not only does it save you a trip to the dealership and a couple hundred bucks in labor, but it also helps you stay in tune with how your machine is actually running. Plus, there's something satisfying about knowing your quad is dialed in perfectly because you did the work yourself.