Cleaning carbeurators on 1982 XS400 -- Day One

My sweetie has a 1982 Yamaha XS400 Maxim. It's a 30-year-old bike and the previous owner was not particularly meticulous in their care and cleaning.

Symptom: It starts properly, though with the usual hesitations of a 1980s carbed bike. But then the idle is really inconsistent: adjust it down to 1500, and it still revs to 3000 or drops suddenly to a stall. The other day, it was particularly bad: it kept bucking and nearly stalling as if it were out of gas, and stalled out completely a few times.

Diagnosis: Uncertain. But last week I swapped out the spark plugs and the starter relay, and cleaned all of the terminals, so I think the ignition system is doing well. Next place to look, is the fuel system and that largely means the carbs. While I'm at it, the air filter is adjacent to that and may need some cleaning.

Goals here: Remove the carbeurators and clean and inspect them, and check and clean the air filter.



Step 1. Put the bike on its center stand, and remove the seat and battery. I put the battery onto the charger for good measure.

The bike on its stand, minus the seat and battery. The gas tank is held on by one bolt.

Step 2. Drain the gas tank. On the left-hand side of the bike, on the ventral side of the gas tank you'll see the fuel cock or stopcock. Connected to the cock are two hoses. The large one is the fuel line, which lets fuel drip into the carbs. The smaller one is the vacuum tube; when the engine is sucking air, the pressure through this tube causes the cock to open. (A quick tutorial: the Maxim has 3 positions: ON and RES both allow gas to flow, but only when there's suction from the engine ; the PRI position allows the gas to flow freely, which is useful for dumping the tank or for re-fiulling carbs after we finish emptying and cleaning them)

• Have a gas can ready.
• Set the cock to the OFF position, if you have it. If not, use the ON position.
• Undo the little clip at the bottom of the fuel line, and wiggle the fuel line off of the metal pipe connecting it to the carbeurator. Expect a few drops of gasoline.
• Put the fuel line into the gas can, then switch the cock to PRI. Gas should flow freely.
• You likely won't get all of the gas. We had a few ounces in the bottom of the tank that just wouldn't come out.

The fuel cock (3) and its connections: the fuel line (1) and the vacuum line (2).

Step 3. Disconnect the various hoses and cables from the carbeurator. They are:

• The vacuum line to the fuel cock.
• The fuel line (which you already disconnected).
• The throttle cable.
• The choke cable.
• For good measure, the cap on the right-hand carb throat.

None of these have a trick to them, really. The hoses wiggle off, but be gentle with them. The cables are like bicycle cables: twist the levers and grab the cables to release some pressure, then put the cable through the opening in the eyelet.

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  Dorsal view (from where the gas tank would be). On the left is the vacuum line (1) and on the right is a cap (2) on the carb throat, commonly used for a squirt of starting fluid.

View of the carb from the right. Note the throttle cable (1) and the choke cable (2).

 

Another view of the choke cable and throttle cable.

Step 4. The block of carbs is held into place by the four boots or throats, one connecting each carb to the air filter housing (where it sucks in air), and one connecting each carb to the engine (where the fuel/air mix is pulled into the engine). The carb sticks into each throat about 1/4" and that 1/4" is tightened down with ring clamps, to keep them from wiggling loose.

The XS400 has 2 carbs, and each carb has 2 throats, which means 4 throats and ring clamps.

The XS400 has 2 carbs, and each carb has 2 throats. That means 4 of these ring clamps (1).

There's no real trick here either, just go at it with your Phillips.

The orientation on them is not important -- when you put these back on later, point the screws whichever way is most convenient for you to get at them again in the future.

It's not necessary to unscrew them all the way until they pop open, in fact getting them back together is annoying. Instead, unscrew them until they dangle loosely around the throat and you're sure that they're not exerting any pressure on the throat.

Having removed all hoses, cables, and ring clamps, the carb is now held in place solely by the pressure of the air filter housing, and you should be able to wiggle the carbs a bit.

Step 5. Let's skip for a moment to the air filter. On the left-hand side of the bike, posterior to the carbs, you'll see a triangular panel. This is the access panel to the air filter. Undo all three bolts (on ours, all 3 were different).

Pull the air filter out of there. It should slide out gently, with minor wiggling.

The access panel on the left side of the air filter housing. On this bike, all 3 bolts were completely different but I'm sure that's atypical.

The air filter. This one is reusable, and wasn't even that dirty.

Check the air filter and clean it if necessary. Most air filters are reusable synthetic stuff, and not disposable paper filters.

To clean an air filter, wash it in soap and water, rinse it thoroughly, then dry it thoroughly at least overnight. Before reinstalling it, give it a light soaking in air filter oil. You can get air filter oil at any car or bike shop. In a pinch some folks use engine oil.

Check the air filter housing and clean it out too. Yours will probably just have dust and dirt. Ours had a mouse nest in it, chunks of fiberglass insulation, mouse poop, bits of paper, and we spent some quality time with the shopvac and paper towels. This air filter was removing mouse turds from the air flow, which is way above factory specifications.

Step 6. To remove the carbs, we need to slide the air filter housing back to release some pressure. To do this, remove the three bolts holding the air filter housing in place AND ALSO the two fender bolts, so the filter housing can slide back an extra inch. You WILL need it.

Two are on the dorsal side, straddling the unishock. One is under the battery cage.

A dorsal view of the air filter housing and the shock absorber. Two of the three (1,2) bolts holding the air filter housing in place.

The battery cage, and the third bolt (1) to the air filter housing.

One of the two fender bolts (1). Left side, posterior to the air filter housing.

Also  remove the two bolts holding the fender in place. This allows the fender and the air filter housing to slide back an extra inch, and you will need it.

Once these are removed, you should be able to pull backward on the air filter housing, and see some significant improvement in the wiggle room around the carb.


Step 7. Start yanking! Have someone pull backward on the air filter housing, to release some pressure on the carbs. Then just wiggle, squeeze, and pull to get the carbs out. It's annoying work, because by design it's too large to fit into the space given!

Step 8. Eventually, you'll get to the empty space in the picture below. Remove the carb throats from the engine with an Allen wrench, and clean them and inspect them for cracks or damage. Clean off the air filter throats too (they don't come off).

The void of the carbs. Clean off the air filter throats (3, 4) and clean or even remove and inspect the engine throats (1, 2).

Step 9. No step 9, that was it. The carbeurator is in hand, and tomorrow we clean it.