Cleaning the carbs 1

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Intro to carb cleaning

If you clicked on this page thinking you were going to find some miracle product you could pour in your gas tank that would solve all your carburetor woes, you were mistaken. There is really no substitute for removing the carbs from the bike and giving them a proper hand cleaning.

The older formula of Yamaha Carb Cleaner used to be recommended in this space, but it is no longer available. Its replacement, also called Yamaha Carb Cleaner, is a water-based product that is less effective than the original. If your online store shows a bottle like the one on the left, they're using an old picture. The newer stuff looks like the right photo.

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The Yamaha Cleaner's main usefulness was always for loosening the crud inside the carbs, making the later (and necessary) hand cleaning a lot easier.

Pre-soaking, for really gooped-up carbs

This is really just pre-soaking. It isn't a complete cleaning. It's still necessary to do the manual cleaning as shown later in this article and in Cleaning the carbs 2. For most carbs, pre-soaking is not necessary, but hand cleaning always is.

Pine-Sol method

Pine-Sol works quite well for really mucked-up carbs.

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Buy a couple bottles of Pine-Sol and mix up 3 parts Pine-Sol to one part water. Pine-Sol doesn't hurt rubber/plastic, so you can leave all the hoses and plastic pipes on the carbs. This will make the hoses more pliable, and you can do the same with the intake manifold boots as well.

You do need to remove and not soak the slides. The slides and diaphragms are all one piece on the 250 carbs. So, remove the 4 screws holding each black top cover, pull those covers, the springs, and then the slides.

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To get the most benefit from soaking, the float bowls, floats, fuel needles, needle seats, jets, and emulsion tubes (the, uh, tube that the main jet screws into) should come out as well. Soak the jets and everything else you removed, except for the slides and diaphragms. You should also work the choke a few times while submerged to get the Pine-Sol everywhere.

Find a BIG container/bucket and some sort of fine wire rack or mesh sieve (like ones for a BBQ grill) that fits inside. The mesh will let all the yucky stuff that falls off settle on the bottom, and you won't have to fish around in the murky fluid looking for one washer or small bolt/screw. Pour enough of the Pine-Sol solution into the bucket to cover the carbs.

Soak the carbs and parts for about four days. Scrub the open areas just a bit with a toothbrush and green scrubby. Then do a normal carb cleaning for the jets and interior passages of the carbs.

The carbs shown at the top of this section, from a bike that had sat outside so long there were chunks of dry-rotted tread falling off the tires, now look like this, and the bike starts and runs well, without a lot of hard work.

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For more information: Read further down this page, see Carburetor photos, EX250 parts diagrams, and look at the other parts of How do I work on the carbs?.

Berryman Chem Dip method

loudboys says that Berryman Chem Dip is the best carb cleaner on the planet. It comes in a gallon can; basically a paint can with an included basket to put the parts in and lower them into the can. He's been using it for 25 years. You can take a carburetor that's been sitting in a junkyard for decades and clean it in an hour. The really great thing about it is that after you're done, you put the top back on and use it again later. A can of it will last for years.

Notice: You have to keep all rubber and paint well away from it (including rubber o-rings, diaphragms and seals). Hard plastic parts won't be affected. You'll want to wear gloves when you use it.

Try Pine-Sol first. It's less caustic and less likely to eat stuff if you keep the carbs in too long.

Stuff you need and need to know

Before you get started, take a look at the instructions for removing the carbs. If you can't follow the FAQ to remove the carbs, then you probably aren't equipped to clean them.


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On the far right is a container of parts cleaner by Next Dimension. There is also a small amount in the Ragu glass jar on the far left. Next Dimension is highly recommended. Next to that is specific Carb Cleaner, by the same company. Spray cans of brake/parts cleaner work just fine as well. You DO need a spray can for this work. The last item, the open funnel/measuring cup, is the old Yamaha Carb Cleaner. Notice that it went from clear to green. It did its job.


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A #2 phillips, assorted flat blade screwdrivers and a length of copper wire. Not shown is the #4 allen used to put back the replacement screws. The copper wire is essential for reaming the passageways and helping clean. Any metal more aggressive than copper has a very good chance of hurting the carb internals and making them not operate correctly. If you just mess up jets, well that's a $5~7 per item mistake, but if you scratch a passageway, then you're looking at a new set of carbs. And it's essential in the same way a toothbrush is, vs. just rinsing with Listerine... you need to scrub the tough bits that are stuck.

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One important thought with regards to working on an engine is that any time you leave an opening to the engine you should block it. If you don't, then Murphy's law will just help you drop something inside. Paper towels are cheap and effective.

Screwdriver notes

It's VERY important that you use a quality screwdriver that's the right size and in good condition. If you don't, then you're going to bugger up the screw and cause yourself more headaches than just buying and using the right tool in the first place. The screws on carbs, brake reservoirs and the 250 bar ends are notorious for being stiff to remove. Don't let something like this happen to you:

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A lot of screws on Japanese products are Japanese Industrial Standard JIS 4633B-3/1991 and DIN/ISO standard 5260, aka "Japanese Phillips". These usually have a dot to one side of the cross slot (above picture). Screwdrivers to fit these are available from Ames Supply Company and Katun, among other places. Using a normal American/European #1 or #2 Phillips screwdriver in a screw head with the dot on it will most likely strip the head on the first try.

A couple other options for float bowl screw removal are listed below. Keep in mind that if alternative screwdrivers have any slop in the way they fit in your screw heads, you should probably have a vice grips handy. Whatever you do, plan on replacing the float bowl screws with allen-headed ones.

  • Drywall driver bits work well for JIS screw heads. Some of our members have had very good results with these, and they're cheap.
  • Try a Pozidriv screwdriver or bit (a #2 in this case). You may find that this fits the screw slots better than a regular phillips, although our man's success was on screws which were 'pre-rounded' by Mr Previous Owner.
  • Take a Dremel and cut slots in the screws, then use a flathead screwdriver to remove the ridiculously soft metal screws. You may have to cut a piece off of the idle adjustment bracket to get at one of the screws.
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  • Carefully put the carbs in a bench vice and remove the screws with an impact driver before you strip them.
  • As long as you're going to replace the screws anyway, use vice grips. Grip the screw top down and hold it hand tight (don't clamp it). Then twist; it should make a popping sound as it breaks free. As soon as it pops, stop twisting, get a phillips, and turn the screws out.

Carb parts

The following are the different parts of a carb. Note that not all carbs look exactly the same. These are NOT EX250 carbs, and not even Keihins, but most carbs are similar to these.

This is a picture of the float. It's what controls the level of fuel in the carbs:

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The floats do this by floating (duh?) on top of the gas and pushing the fuel needle against its seat. The fuel needle (shown below) should be inspected to ensure that the rubber tip is a well defined cone. If there's a dish in the middle, that means it's worn out and needs to be replaced. Worn needles will leak fuel into the carbs, which will then leak into the engine and flood it, as well as leak past the rings and contaminate the oil. Just a whole ball of wax best to avoid:

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This is the main jet:

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The pilot jet:

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And the enrichener (choke) pickup:

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When you take out the main jet, the emulsion tube (called 'holder' in the left picture) is behind it, with the jet needle (also called 'collar') behind that. Be sure you get these little parts out (part #16017 on the diagram), as they could fall out on their own if you don't.

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When you replace the collars, check in the carb intake to make sure they're poking through. They only go in one way. The longer, narrower end goes in first.

For more views of all these parts, see Carburetor photos.

The cleaning process

Remove all of the above items prior to cleaning, except the enrichener pickup; that's part of the carb body. This should leave you with just an empty-looking float bowl (the bottom part of the carb). This is the 'meat' of the carb. Up top is just the slide and needle (which will be touched on later in this article). You need to probe each and every hole with some wire. Twist a few strands together for the big holes, and just use 1 piece, slightly kinked up, for the smaller holes. You want to be sure to get ALL of the holes, like these on the side of the pilot jet:

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The pilot circuit, also called the slow speed circuit, has the smallest holes/passages. They are the first to clog and are deserving of the majority of the attention. The pilot circuit outlet:

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And the transition outlets. These allow for more fuel to enter the engine than just the pilot and help smooth the transition to the midrange, which is controlled by the needle. These aren't pointed to, but are visible just under the brass butterfly valve in the center:

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Take the compressed cleaner and, using the straw, squirt fluid into every hole. You should have cleaner going into or coming out of each and every hole, though not all at the same time, and in the same volume as the other carb bodies. And it should be coming out clear (this one started greenish).

You should also consider cleaning up the fuel needle seats. The fuel needle fits into the big hole shown here. The tip fits into the small hole at the bottom and forms a seal. You want to clean the edge of that hole, so there isn't any crud that can cut the tip or hold it up (so it won't seal). Either of these will cause fuel to leak past.

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To do this, cut a Q-Tip in half in the middle of the stick and put it in a dremel. Put a little dab of Simichrome metal polish on the end and let it run at full speed for about a minute. Use the other end of the Q-Tip and carb cleaner to clean up the left-over gunk.

When it comes time to reassemble, the jets just screw back in, but you need to set the float height. Each set of carbs has a different recommended height (listed in the service manual for the given bike). If the floats hang too low, your bike can (and most likely will) run lean, and if they sit too high, you'll likely be fighting a rich running bike. The picture shows a special tool for measuring float height, but a 6" ruler, or even a basic set of calipers, can be used to measure the height. To measure, you want the floats to just be touching the fuel needle, which has a spring on it - you don't want to compress that. Most carbs can just be set upside down and the spring will hold the floats where they need to be; some carbs you'll need to tilt at an angle while doing the measuring. To adjust up or down, you just bend the small metal tang that rests on the fuel needle:

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With that set, feel free to screw the float bowl back onto the carb. It's best to replace all the phillips screws with stainless allen screws, as they're easier to work with and aren't too expensive. Float bowl screw size is M4 0.7 pitch, with 14mm length. This is commonly written as M4x14 0.7, although you may want to bring your old ones with you, just to make sure. If you have to get slightly longer screws (say, 16mm) that will work fine.

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Up on top you have a diaphragm that controls the speed at which the slide goes up and down via high and low pressures, which raises the needle, which directly controls the midrange fuel delivery. The needles are traditionally set a bit low to lean out the bike for EPA emissions. The most standard fix is to just put a couple 2.5 or 3mm washers on the bottom, so the needle is raised up a little from the start.

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That's the only adjustment that can be made up top. Make sure that the diaphragm doesn't get damaged or pinched when you reassemble. The carbs won't work right, and they are expensive to replace! Test all the slides by just pushing them up with a finger and comparing them to banks you haven't touched yet, or just make sure they all react the same.

That's all there is for cleaning and adjustment inside the carb. There is one other adjustment outside of the carb, and that's the air/fuel mixture screw. This is the item that has the plug covering it from the factory, which is usually called an anti-tamper plug or "EPA plug". You need to drill a small hole in the plug, then screw a screw into it and pull the plug out. It's just a pressure fit. What you'll be left with is the screw that meters the air OR fuel for the pilot circuit. If the screw is on the engine side of the carb it meters the fuel; if it's on the airbox side it meters the air. Again, this is set to the lean side from the factory. By adjusting this screw, you can help cold start issues. A good base setting is 2.5 turns out from fully seated:

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And that's it for what can be done on the bench. When you install the carbs again, do a carb sync.

Here is the full gallery of photos used in this write-up.

For another look at carb cleaning, with even more pictures, see Evan Fell's blog