® Carburetor Rebuild Techniques

Okay, here are some things to look out for when seeking either how-to advice for rebuilding your carburetors yourself, or a rebuilder for your carburetors. Hopefully, this will keep you out of too much trouble, as most of these tips regard things *not* to do.

Idle Jets
First, with so many late model Keihin motorcycle carbs having pressed-in idle jets, it is important that, whether you or someone else does it, those jets be removed for cleaning, and cleaning not only the jets themselves but the intricate passages underneath. There are some good and some not so good ways of going about removing these jets, and if done properly they are reusable. Something to bear in mind is that on VB series Keihins the jets are recessed so drilling and tapping is needed to extract them, which produces chips and debris. Therefore, the carbs must be thoroughly flushed *afterward*, meaning that a set of multiple carbs has to be unracked for proper cleaning out of the debris, when removing the idle jets.

Primary Main Emulsion Tubes
Also on VB series Keihins, early examples have what is called a primary main circuit. This circuit includes a tube that is difficult in many cases to remove. Many have resorted to using screw extractors ("easy outs") but this is not a recommended method and not the one used by professional rebuilders. Beware of the rebuilder who shows a lack of understanding in this area, and look out for those who completely give up and neglect to remove this part during their rebuild.

Aircut Valves
I have written on this elsewhere, but Mikuni and Keihin carburetors having aircut valves often trip up the unwary. First, be aware that the aircut circuit is not an emissions part, and is not even a necessary circuit. It was added-on through politics more than anything else. Learn more about this in my article Keihin Ide Circuits White Paper. Remove the aircut diaphragms before beginning cleaning work on your carbs, or make sure the person doing the rebuilding does. The aircut valves must be disassembled during a rebuild because the circuit is part of the idle circuit, the most critical circuit in any rebuild due to its small size and high complexity.

Float Valves
Using badly manufactured aftermarket float valves continues to be one of the most serious mistakes a rebuilder can make, whether a do-it-yourselfer or pro. The limited availability of factory parts and their high cost drives many to the aftermarket, but in this case as in many others, it is almost always a very poor choice. Aftermarket float valves seldom seal as well as the original for one thing, the very reason you are replacing the valves. I test each one I am forced to use (only on very old early 70s carbs am I forced to use aftermarket valves) and 50 times out of a hundred they leak, meaning I often have to use two sets to get one good one. For late model (78-90) Keihin carbs the situation is better because these valves are still available from the factory, albeit at *very* dear prices. Resist the temptation to go with the much less expensive aftermarket valves for these carbs. They are some of the worst parts in the carburetor parts industry, made of zinc instead of aluminum and chrome plated. They're easy to spot and are not worth your time.

Float Valve Seats
Though they can present some problems, in most cases float valve seats are non-maintenance. On some VB series (early Honda DOHC four, for example), they are even pressed-in, making servicing them a moot point. Careful cleaning with carb aerosol and a Q-Tip is all they need and should get. Beware of folks who put *anything* other than carb cleaner inside float valve seats. Under no circumstances should the seats be burnished or polished, as this has the potential for changing their internal shape, ruining the whole carburetor (because the seat is pressed in). Aftermarket seats often include poorly made filter screens that look quite different from stock and don't fit as tightly. Also watch out for sealing washers that are thinner than stock, quite flimsy in fact, as these wear quickly, interfere with the filter screens, and alter float height setting.

Other Attempts at Sanding or Polishing
I suppose this is controversial, but in my book the only parts that should be sanded or polished on a carburetor are the covers. That is, the tops or bottoms. That's it. Not the slides or slide bores, not the butterflies (!), and not the float valve seats (see paragraph above). These things are manufactured to a high tolerance. Do you go sanding the valves in your engine, or take apart your watch and start sanding on things...?

Epoxy Repairs
Gasoline is pretty funny stuff (and getting more so all the time). It doesn't mix well with most glues and epoxies. In fact I know of none that resist it long term, not even marine epoxies and Devcon F. If someone has done epoxy repairs to your carburetors, or is planning to -- don't. And when it comes to the ubiquitous JB Weld (a common epoxy), consider this. Readily available carb cleaning chemicals dissolve it. That's right. Dissolve. As in not a trace. There are much more professional ways to do these kinds of repairs.

Carburetor castings are made of an aluminum alloy that is not very "clean;" there is a lot of iron and zinc in the mix, especially in pre-1980 carbs. The metal is already ripe for electrolytic action, don't make things worse by adding stainless steel screws. Faster than you can say "electrolysis," those screws will begin bonding to the casting, and the next time they are removed they will take aluminum with them, tearing up the threads in the carburetor. Sure, those stainless screws are pretty, but at what price? Yes, a dab of grease on the screw threads can help, but overall the use of stainless screws just isn't a good practice. Worse yet is most of these screws are socket head (we call them Allen head, but that is actually a trade name like Kleenex). Allen screws permit too much torque to be applied to the threads, especially if using a typical Allen wrench. If you must use Allen screws, at least avoid the L-shaped Allen wrench and use instead a screwdriver type. You can tell an inexperienced carb rebuilder by his use of these kinds of screws. While on the subject of screws, it is time to speak of screwdrivers. If doing your own work, you really should invest either in mechanic's quality screwdrivers or if you're not sure what that is, then hunt up some JIS screwdrivers, which are sort of sanctioned mechanic's grade tools. They're a lot easier on your Phillips screws.

Bench Syncing and Other Final Adjustments
Especially if the carbs have been unracked during the rebuild, which is in most cases a given, the throttles will need to be resynced, and this should start with a bench sync. The accepted method is to open one throttle until the butterfly just uncovers or bisects one of the carb's bypass ports. Under no circumstanes should anything be drug under the throttle butterfly! Not only is this a method for a different type of carburetor altogether, it is harmful to the butterflies, which are finely machined and very soft aluminum. A few rebuilders test your carbs on a running engine, and that's a pretty good thing to do. But it's good more from the standpoint of the rebuilder catching his own errors, such as leaks, than for other reasons. Working well on a test engine in no way guarantees your carbs will work equally well on yours. All conscientious rebuilders test your carbs in many ways. Leaks, accelerator pump action, fast idle mechanism, etc.

Idle Mixture Screw Limiters
Being a longtime, factory-trained, in-the-trenches for years seasoned Honda tech, I get annoyed and a little saddened each time I start a rebuild on a set of carbs and find that they still have their factory-installed idle mixture screw limiting parts on them. Imagine it! 30 or more years without a tune-up! What kind of shop would allow this? What kind of owner? Whatever folks are being told on Internet forums, the limiting covers and stop flags on idle mixture screws are supposed to have been removed, if not during the initial setup, then at least by the time of the first maintenance service. So I take a very jaundiced view of any carbs that still have them. How can you adjust them properly during a maintenance service? On VB series Keihins, the flags are superglued on and come off easily with the application of heat. On VD series Keihins and most later Mikunis, one has only to drill the aluminum over and pull it out. The experienced tech adjusts the idle mixture by ear (please see my Keihin Idle Circuit White Paper). Less experienced folks should use an exhaust gas analyzer (EGA), although leaving the screws at the optimum starting point of 2 1/2 turns works fine longterm on VB carbs.

Thread Locker
There is only one place anaerobic thread locker such as the ubiquitous Loctite belongs on a carburetor, and that is the choke plate screws, if applicable to your model of carburetor. But beware, even here, the unprofessional messes up. Never, never, use a medium strength or stronger thread locker on those tiny screws.

Throttle Shafts
In most cases, carb throttle shafts should not be removed. I expand on this subject in my article Carburetor Myths, but the short of it is this operation is too delicate for most to fool with and is very seldom necessary. The felt seals most carbs have on their throttle shafts will live happily a very long time as long as folks do't mess with them.

Fuel Line
In my book, the presence of plastic fuel line on a carburetor is a sign of unprofessionalism. That stuff is so crappy it shouldn't need to be explained, it should be obvious. Only if I were doing a concourse level resto on an old Brit or Italian bike would I even consider using the stuff.

Carb Kits
I have had a lot to say about carb kits in my Carburetor Myths article, so sufficient to say they are junk and another way in which would-be rebuilders drop the ball deeply into the weeds. Read the article. Keyster, Napco, BikeMaster, Sabre Cycle, or K&L rebuild kits -- they're all trash. I can think of few ways to muck up a good set of carbs worse. If your bike is covered by Randall Washington's selection, you can do no better. Great stuff. Better than OEM. The problem with carb kits is twofold. One, they contain the aforementioned float valves that either don't work at all (I used to test every aftermarket valve before installing and a consistent 50 percent failed) or they fail within a short time, often a year or two. Sure, factory valves are expensive, as in four times the cost of aftermarket. In this instance however you really do get what you pay for. Second, carburetor kits are invariably filled with a lot of brass parts. There are a number of problems with this. You very rarely need to replace jets in a carb rebuild. Worse, they usually made to specs that vary from stock. And worst of all, being made of brass they will wear much faster than the original materials on U.S.-spec Keihins made after about 1977.

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