Rebuild Your Carb

It's not that hard unless you're Stupid

Apr. 01, 2007 By Rick Sieman
Carburetor rebuilding. It 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. It’s a great excuse for those times when you’d rather not do something un­pleasant.

Ninety-five per­cent of the time, carb rebuilding is nothing more than taking the thing apart, cleaning it all up, replacing a few worn parts, and putting it back together. A thorough, immaculate cleaning job should not take any longer than an hour to per­form, and more like 15 minutes if you’re in a hurry.

The two major types of carburetors available are Keihin and Mikuni.  On older (vintage) bikes from Europe, you’ll normally find Bing and Amal carbs.  Older Japanese bikes usually have the round slide Mikuni, but all later Mikunis are the flat slide types. 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, some Mikuni parts fit other carbs, 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.

First, remove the carb from the bike. In most cases, it’s easiest to remove the top of the carb and pull the slide out before you pull the carb out of the manifold and air-box boots. This is fine—just leave the slide hanging there from the cable and take the carb over to the bench.

Drain the old fuel out of the carb body and remove the four 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 seat. Remove the main jet and pilot jet; push the needle jet up with a blunt object and pull it out the top of the carb.

Moving up to the outside of the carb, re­move the idle adjust screw and the idle air screw. Remove the choke lever, and care­fully remove the nut that holds in the choke assembly. Pull the choke assembly, spring, and related parts out of the body, and sit them on a clean rag with the rest of the jets.

A good example of a slightly worn slide. If the grooves get any deeper than this, it will be necessary to replace it. The new one will last longer if dirt is kept out.

The body is now stripped. Take the body and submerge it in a bucket of solvent )or carb clearner) and scrub it down with a small (clean) toothbrush. 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 cable; clean them, too. As you remove the parts from the solvent, lay them out on a clean rag.

Your basic choke mechanism. At the very bottom of the plunger is a rubber gasket—if it wears out, it will give you grief.

No matter what your friends say, the main jet and the pilot jet cannot wear out. Just blow them out with air or a spray of contact cleaner, and set them aside. Never clean them by running a wire through the holes—this can wear them out and make them useless.

Check the throttle slide, needle jet and jet needle for wear or scuffing, and replace them if they look at all worn. Separate the float needle and its seat, and inspect the tip.  If there’s any indication of wear, get a new setup.  It’s cheap and critical to prevent leaks.

The Mikuni is the most popular carb on today’s engines, and probably the easiest to get parts for.
Ninety percent of carb rebuilding is nothing more than simple cleaning. Al­though it is possible to clean a carb body in soapy water, we suggest removing the sponge before reinstallation.
An old toothbrush makes a good carb cleaner.  Start off with warm water and soap and scrub away.
Jets can be cleaned with carb or brake cleaner spray cans.  Here the main jet gets a spritz.
For proper cleaning, remove the main jet from the needle jet. 
The needle jet can be removed by pressing it out with a small screwdriver handle.
Normally four screws hold the float bowl in place.
Float bowl removed.
As long as you have the float bowl off, check the drain plug for crud and clean it thoroughly.
To get to the pilot jet, you normally have to remove the main jet and main jet baffle.
A small flat bladed screwdriver removes the pilot jet.
Pilot jet removed.  Spray clean the small jet hole thoroughly.
Remove the pin holding the float tangs, then set if off to one side.
Then the locking nut and the tab on the float needle can be reached.
Tab removed.
Remove the small spring holding the float needle in place by gently prying it off with a small screwdriver.  If the float needle or the seat is worn or badly grooved, replace it/them.  Clean them thoroughly before replacing. 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.
The choke assembly is either a plunger affair, or a lever.  On the plunger type, a 14mm wrench loosens things up.
Choke assembly removed;  check for bad o-rings, seals, or crud. 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.
Gaskets, such as these on the float bowl, should be replaced when they’re hardened or damaged.
Screws hold the top of the square slide carb.  On the round slide carb, it’s a threaded top.
Top can be taken off with the slide attached.  Most slides are spring loaded.
The slide should be checked for wear and scratches and replaced if needed.  Clean the inside of the housing and the slide itself.
While you’re cleaning the carb, check the petcocks for bad seals, o-rings, or contamination.
Bing carbs can be found on many of the European and older bikes. Although the parts look different, they serve the same function as those on the Mikuni and Keihin.

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.
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 needle.

Rebuild Kits     Seals and Gaskets.  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, and button everything up.  Congratulations, you have now rebuilt your carb; and no... you don’t have to tell everyone how easy it was. Newsletter
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