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Common Tread

How to Clean a Carburetor 101

Mar 25, 2014

Up on Lemmy Mountain, favored fall activities include drinking beer and hunting. Unfortunately, fall also usually heralds the end of riding season, so before I pop a top or sight in the rifle, I make sure my bikes are winterized and "put up" in true Lemmy fashion.

I’m diligent about winterization because I got sick and tired of wrenching on all my bikes on the first nice day of ridin’ season. I’m not the only one to make that mistake, either. I still do lots and lots of carburetor clean-outs for friends. So if you parked your carbureted bike in the garage last fall, intending to winterize it later, and spent all winter watching football or playing Call of Duty instead, here’s what you need to know to get your neglected bike running again for spring.

How to clean a carburetor

  1. Turn off the petcock
  2. Drain the fuel bowl(s)
  3. Remove the bowl(s)
  4. Clean the bowl(s)
  5. Remove the jets
  6. Clean the jets
  7. Reassemble with fresh gaskets

Why do it yourself

I was inspired to write this article by a friend and customer who needed to get his bike up and running for this season but was a little light on loot. I’m going to let you in on a shop mechanic’s secret: If a mechanic is going to charge you a few hours of labor to remove your carbs and clean them, you will be paying for a stem-to-stern overhaul. Why? Because he doesn’t want to do the job twice. You won’t want to pay for his labor twice, so he’s going to make sure to clean everything in there the first time.

A complete carburetor rebuild is a topic for a more involved article or a visit to the mechanic mentioned above. On the other hand, if the only problem standing between you and getting on the road on the first warm day of spring is some old gas gumming up the works, a light-duty cleanout very well may be all you need to get rolling. As with all free advice, this is worth what you’re payin’ for it, but it could help you get on the road with an afternoon’s effort and limited expense.

Opening considerations

Let’s begin with some theory. First, you don’t need to know everything about how a carb works, but some background would help. Fuel enters the carb (usually) from the bottom, into the fuel bowl. The engine vacuum then sucks the fuel in a fine mist through metered holes called jets to deliver a precise ratio of air and fuel to the engine.

Particularly when working on an older bike, you need to be extra-mindful of your bike’s normal starting routine. It differs from bike to bike and carb to carb, and what works on one bike may be way off for the exact same setup on a different bike. If you don’t know by heart how many prime kicks, prime wicks, and how much throttle (if any) your bike requires to start, you’re going to beat up your battery, if you have one, or your leg. Invest in a full rebuild if you aren’t sure what your bike usually “likes.”

If you didn’t treat your fuel or empty your carb before storing the bike for the winter, I am betting you left your poor battery sitting out in the freezing cold all winter, too. Plus, you probably already tried to start the bike 600 times, praying all the while that she’d fire up, and that’s how you found out the carbs are gummed up. As a result, the battery is shot, at least temporarily. Charge it, test it, and replace it if necessary. Do not skip this step.

Next, if the fuel in the tank is bad, drain it and refill it. Oh, you can’t tell? It’s only a few gallons. Drain it, pitch it in your car, truck or lawn mower or discard it and refuel with fresh gas.

Is all this stuff a pain? Yes, but so is hauling your bike to a mechanic and paying him a bunch of money, right?

Here’s one more piece of advice before we dig in. The steps in this article will work for any carb, but if you’re riding a bike with hard-to-remove carbs, like a Japanese inline four-cylinder, or worse, a V-4, pull the carbs and do a full breakdown on them. The pain of removing them is so terrible, you’ll pull your hair out if you have to do it several times. I’m approaching this from a Harley, single-carb standpoint. I can have one of these off a bike in six minutes if I have tools in front of me and I can usually do it with a beer in one hand.

Clean up, you slob

Now, it’s carb cleanin’ time. Let me tell you another mechanic’s secret: 90 percent of all carbs that won’t feed fuel well enough to run just have crud in the jets. Air passages and such usually don’t clog over something as simple as a two- or three-month winter break.

First things first, you technically need gaskets. Technically. Usually paper and rubber alike can be reused, if you are careful. Really careful. So, for the record, I encourage you to use new gaskets. I have slapped together enough roadside fixes, however, that I can tell you they are not always 100 percent necessary. Be careful when removing them if you want them to work when you reuse them.

You can leave your carb on your bike if you’ve got room, but everything’s easier to do on the bench. This carburetor we’re about to tear down for pics here is an old Super B. They’re easy-peasy to clean while on the bike because they hang approximately 60 feet off the side of an old Harley. It’s hard to take photos of carb guts from that angle, however, so I removed it for your viewing pleasure. If you are attempting this on, say, a Hinckley Triumph, do yourself a favor and yank the carbs off the bike. If you are uncertain whether you should pull the carb, pull the carb. Don’t booger up expensive internal carb parts (or the body itself) just to save yourself the pain of removing a carburetor.

Begin with your trusty petcock turned to “off.” (You did empty that gas tank like I mentioned earlier, right?) Loosen the push and pull cables on the throttle and disconnect them from the carb. Unbolt the carburetor from the manifold and bring it over to the bench.

Drain the bowl. The bowl is the bottom of the carb and it usually is shaped the way its name implies. All the bowl does is hold fuel -- and gunk. On the very bottom of the bowl, you will see a drain screw. Loosen it! Fuel may or may not come out. If you have a hose on the bowl, the fuel will exit from there. If you don’t have a drain hose, I recommend putting one on.

This carb you are looking at belongs to Crash Strader, one of our Gear Geeks. He’s got an adjustable main jet stuffed into his bowl, but the concept is the same. Loosen the large nut (top right), and fuel will come out. After emptying the fuel in the bowl, retighten the drain screw and flip the carb over. Loosen the four screws that hold the bowl on. In this photo (lower right) they are the four flathead screws. A few pointers -- if you are breaking apart a Japanese carb, most use JIS fasteners, not Phillips-head. If you don’t know the difference, it’s OK, but use a JIS screwdriver (you’ll have to buy one from a specialty tool shop), or be prepared to replace those pieces of hardware after you strip them with a Phillips screwdriver. Hex-head cap screws make excellent replacements if they do indeed strip. (They seem to strip often. The metal is usually quite soft.)

Next tip: For some carbs, like the S&S we are working on here, extended bowl screws are available. They are comically large, unbelievably expensive for what they are, and so damn convenient you won’t mind the size or cost. They’re knurled so you can just spin them off by hand. I can do a jet swap on a bike equipped with these in about five minutes when tuning a bike, and that’s a good indicator of how awesome they are.

Here is my final tip for this section: Once you get the screws out, you will be tempted to separate the bowl from the carb body by yanking. Don’t! Remember those gaskets you were too cheap to buy? Tugging aggressively at the bowl is the perfect way to rip and ruin them. If the bowl doesn’t slip off easily, take a little rubber mallet and tap it gently. It will come off. If you didn’t go caveman-style on it, your gasket should look something like this (right) after you slide it off the body. (I’m referring to the level of non-destruction, not the shape. Japanese carbs often have bowl gaskets that are open in the center.) In the photo below, you can see the gasket still in place.

You now should be staring down into your bowl (above). The float (the black round thing here) may be on the carb body or in the bowl, depending on what type of carb you’re working on, but leave it alone. Work around it, and be gentle with it. If you make one little pinhole, your float won’t float and your bike will not be a happy camper. If the bowl has crud in it, get it out. Clean it with some carburetor cleaner or kerosene and wipe it clean. That gunk is what’s plugging up your carb’s jets. You want to get out all the scummy stuff like you can see in the photo above.

You now will want to remove the jets, as indicated here (above). Be careful! Most jets I see pass through the shop are mangled because our favorite caveman couldn’t be bothered to find the correct hollow-ground screwdriver or, better yet, the fancy S&S tool to remove jets. The main on a Super B comes out with a screwdriver or S&S jet tool and the pilot jet comes out with either a flathead screwdriver, the S&S jet tool or, very carefully, with pliers wrapped with a rag. Be careful, remember? You can also remove the discharge tube, too. That’s the brass piece the main jet screws into. Here (above right) you can see the jets out, un-mangled.

Now we need to clean the jets. Some people use solvent. Some people like compressed air. Some use mechanical methods. The safest is solvent, if you have the time and chemicals. But, if you’re readin’ this, you don’t. There is a backyard way to do this, but like all the others, it requires being careful! (Have I stressed that point? When you’re doing things the wrong way, you have little margin for error.)

I play guitar and I’ve found a good use for an assortment of all my old strings: jet cleaners. You could also use a torch tip cleaner or very fine mechanic’s wire to clear the orifice. The “careful” part means no scratching, sawing, poking or drilling. The jet is a super-precise piece of equipment. If you foul it up, your bike will run like doody-poop, for lack of a better term. Just so you get an idea how tiny these passages are (and why they clog so darn easily), here (above right) is a picture of an assortment of domestic and Japanese jets.

Because Japanese bikes typically have multiple, small carburetors and smaller displacements relative to a Harley, they tend to be easier to clog and more difficult to clean. Just take your time, be delicate, and make sure, when you think you’re done, that you can see a nice, round, open orifice. If the passage does not appear round or you cannot see light through it, it’s likely not clean. Clean the holes that sometimes run across the jet, too.

Now, reassemble your carb. I know I just reversed the directions on you in one word, but if you made it this far, reassembly is a breeze. Reinstall the discharge tube, put in both jets, set the bowl gasket in place, put the bowl on, and reinstall the carb.

At this point, fill the tank with the fresh gas I told you to get. Hook the motorcycle battery up to a car or truck battery so you don’t beat it to death. Turn the petcock on and wait a moment for the fuel bowl to fill. If you have an accelerator pump on your carburetor (Keihin CV and S&S Super E come to mind), give the bike a prime wick or two. At this point, start kickin’ or spinning the electric foot. Magic should happen. (Unhook the donor vehicle’s battery quickly after your bike pops to life.) A best-case scenario is that the bike fires right up and you go ride. If that happens, stop reading. Go putt with your bros. You’re welcome. Don’t forget to winterize next year.

If you can get it to fire up, but it’s not running quite right once it warms, that’s OK. Pour a good fuel solvent into the tank. I make up my own brew, but there are some great in-tank cleaners for sale commercially, such as SeaFoam or Marvel Mystery Oil. You’re going to do an Italian tuneup once the fuel treatment is in the tank. However, instead of de-coking the bike (the main goal of the Italian tuneup), you want to create a high vacuum situation in the manifold to pull gas through the carb. High vacuum is best created by running the engine fast under load. Because carburetors operate by drawing fuel in under vacuum, the act of riding uses the carb to pull all that solvent through the tiny openings, clearing away the hidden crud your guitar string missed and getting it acceptably clean. This does work, and after a few minutes of hot, hard riding, bikes often run fantastically.

There you have it. If you don’t mind a little work and a few funny smells, you can usually get your sled rolling again on the cheap and have a few nickels left to rub together for beers.

Now, those of you observant and wise enough to have learned your lesson are wondering how you can avoid this fate next year. It’s all in the prep before you tuck your bike away for a long winter’s nap.

A Lemmy-style winterizing involves topping off the tanks and putting stabilizer in the fuel, turning off the petcocks, changing all fluids, spraying a shot of fogging oil down each jug, hooking up a battery maintenance charger and, finally, gently laying a nice soft blanket over the bike. This effort on a day of lousy weather at the beginning of winter will spare you at least as much time wrenching on that first nice day of good riding weather in the spring, when you’d really, really rather be out on the bike.