In the column I do every month, I often give the generic advice to an owner of a puking or blubbering bike to "rebuild your carb." I take it for granted, because it seems like a simple thing to do, and one that I've done literally hundreds of times.
Therefore, it's only fair to pass on the "how-to" stuff to those who need it. And who could need it more than owners of older bikes. Especially people on a budget who buy older bikes and want to get them running right again.
 Most of carb rebuilding is nothing more than simple cleaning. Although it's possible to clean a carb body with soapy water and a sponge, we suggest removing the sponge before reinstallation. |
Carburetor rebuilding sounds like such a high-tech job, something that would take the better part of a day and require dozens of precision instruments and months of specialized training and experience.
In actuality, all it requires is a bit of patience and a nice, clean working area.
Your friends never have to know how easy it really is, and you can dazzle them with your skill, but ninety-five percent of the time, carb rebuilding is nothing more than taking the darn thing apart and putting it back together.
DIFFERENT KINDS OF CARBS
On most modern bikes, you'll find only Mikunis or Kehin carbs. However, if you're in to vintage or classic bikes, there are a whole bunch of mixers to consider.
 Here's an example of a slightly worn slide that's probably re-usable. If the scratches or gouges are worse than this, replace it. |
The three major types of carburetors available are Bing (KTM, Maico, most European bikes), Keihin (Honda), and Mikuni for most other Japanese bikes. You'll also find Amals, IRZs and Jikovs on many of the older Euro bikes.
Each one of these five different brands differs in construction, but they all perform the same function and require the same type of maintenance.
Naturally, Bing parts do not fit a Mikuni, Mikuni parts do not fit a Keihin, and Keihin parts fit nothing but a Keihin. If you need parts, gaskets or whatever, take the pieces in question to your dealer and replace them with fresh parts.
DOING IT
First, remove the carb from the bike. Normally, you would remove the top of the carb, pull the slide out and let the slide stay attached to the cable. You can spray a bit of carb cleaner on that later, but for now, just take the carb body over to your work area.
Drain the old fuel out of the carb body and remove the screws that hold the float bowl on. (It'll be held on by a spring clip on the Bing.) Remove the floats, and the float needle and the seat.
Next, remove the idle adjust screw (if it has one) and the idle air screw. Take off the choke lever and carefully remove the choke assembly, with the spring and all the related parts.
 The Mikuni is the most popular carb around and the easiest to get parts for. |
All the jets come out last. I like to set everything on separate rags on my work bench: the jets on one rag, the choke assembly another, and so forth.
The body is now stripped. Take it and submerge it in a bucket of solvent and scrub it down with a small (clean) paintbrush. Clean off every speck of dirt and crud, inside and out. If you want to get the body totally shiny-clean, dunk it in a bucket of commercial carburetor cleaner. It'll come out looking brand new. Carb cleaner is expensive, however, and you don't really need it. Just make sure that the body is perfectly clean.
Once you're satisfied that the body is clean, do the same thing to the loose parts, taking care not to lose any of them. Remove the top of the carb, the spring and the throttle slide from the end of the throttle.
REPLACING PARTS
No matter what your friends say, the main jet and the pilot jet cannot wear out. Just blow them out with air and/or contact cleaner. Make sure the hole is completely clean. If you peek through it looks less than a perfectly round hole in the jet, it has a build-up of crud in the jet hole. Don't clean the jet holes by running a wire through them, as it can enlarge the holes.
 Here's your basic choke assembly. At the bottom of the plunger, you'll find a rubber seal. If it wears out, you'll have problems. |
Check the throttle slide, needle jet and jet needle for scuffing or wear, and replace them if they look at all worn. Separate the float needle and its seat, and inspect the tip of the needle for wear, If it has a groove worn in it around the tip, replace the needle and seat as a unit.
The choke system consists of a metal plunger with a rubber gasket at the bottom. Replace the choke plunger if it looks excessively scored, or if the rubber gasket is worn. Treat the idle adjustment screw and the idle air screw the same as the float needle: If the tips are worn to any noticeable extent, replace them outright. Check all related 0-rings and rubber seals; replace them if they look bad.
REASSEMBLY
Basically, put it all back together the way you found it. Snug all the parts down with a wrench or screwdriver, but don't apply excessive force-just slightly tighter than finger-tight will do.
 Bing carbs can be found on many older European bikes. Even though the parts look different than the common Mikuni, they serve the same basic function. |
Check your float level as per the instructions in the service manual for your bike. Usually the floats should sit parallel to the carb body, with the tang on the float assembly just touching the spring pin on the needle. Do the same thing with the idle air screw (most manuals call for an adjustment of one and a half turns out after lightly seating the screw).
If your float bowl gasket was leaking before you started the rebuild, replace it with a new one.
Finally, reattach the cable to the slide, drop the slide in with the cutout on the bottom facing the back of the carb, screw the top on the carb, and clamp it back in place on the bike.
You have now officially rebuilt your carb; and no, you don't have to tell everyone how easy it was.
Source: Dirtbike.off-road.com
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