The Keihin 40mm CV Carburetor

Some Keihin CV Carb History, and Design Specifications

Keihin is a well known carb maker in Japan, that has been making CV (constant velocity) carburetors for motorcycles for many years. Unlike "slide" carbs (similar to Amals, or Mikunis), which can be finicky, hard to tune, and generally offer primarily top end performance as their specialty, the Keihin CV carb is designed for better all around drivability. Honda has used them exclusively since the 1960's. As you know, Harley-Davidson (H-D) struggled for years with poor carburetors until 1988 when they had Keihin design the 40mm CV unit that was used from 1988 through 2006 (even later for Sportsters). It comes on everything from their little 500cc Buell Blast, the Sportsters, and all of their Big Twins. The only differences are in jet size, and fuel needle choice. They have installed millions of these carbs on their bikes with great results. Indians are very similar to modern H-D's, where carburetion is concerned, so the Keihin CV is a good choice if the rider wants to step up to more power, smoother running, easier starting, less cylinder heat,and better milage than the original Linkert carb. They used to be available, and relatively cheap, but the sources are drying up. It seems that most are on E-bay, but be careful what you are buying. Now, they are being reproduced in China, and are very affordable. They do have some problems that can be fixed (not unusual for anything from China). I have solved ALL of these problems, and I have posted a page covering the fixes. Otherwise, they are exact copies from the original tooling. I can do all the fixes, and the cable bracket mod for $75.00 if you send me the carb.

These carbs can be identified by their large black plastic diaphram cover on top. They have a 40mm throttle bore, and are equipped with both a butterfly, and a slide with the fuel needle that is lifted by "flow induced" vacuum entering the top of the diaphram. There is a low speed pilot jet, a main jet, an idle mixture adjusting screw, a choke enrichment system, an accelerator pump, and a mid-range fuel needle in the slide. All of the carbs share the same body characteristics, so any carb you find will work the same. There usually is a stick-on tag where the last (2) digits are the year of manufacture. The very early carbs had no accelerator pump (1988 Sportster only). Avoid these models, because the pump is a nice feature. Look for the brass spray nozzle in the mouth of the carb! These carbs normally come with a short Choke cable, that will attach to my replacement Indian air cleaner back. If this choke cable is missing, the parts can be easily had from your local H-D dealer for around $44.00 for all of the parts. The Choke cable has a large nut and lock washer. Be careful to only tighten these by hand, because the cable is plastic, and will easily break if a wrench is used! Next to the knob on this cable is a knurled tension ring for adjusting the tightness of the tension on the cable. Turn it in or out to adjust how the knob stays in position as it is operated. There is a small vacuum port at the top and rear of the carb. This is for H-D ignition retard. Plug this with the little black rubber plug included in my kit. These carbs are totally leak proof, and under the float bowl is a small drain port for breathing, and a 10" rubber hose should be attached and routed between the primary, and engine case to the ground to drain off any errant fuel.

Under the carb is an idle mixture screw recessed into a hole that must be exposed by removing a small disc shaped plug to facilitate mixture adjustment (carbs that I sell, or rebuild will have this disc already removed). Drill a small hole in the small flat plug, and screw in a small screw to pry out the disc. Be carefull not to drill too deep! The adjustment screw is used to find the fastest, and smoothest idle speed when fully warmed up. This is usually 3 1/4 turn out! This screw meters idle mixture from the pilot jet through (1) small hole in the carb's outlet from the pilot jet. The factory pilot jets are always #42. These must be replaced with a #45 in all cases, and are available from your local H-D dealer. This pilot jet feeds fuel for low speed through mid-range running through a series of (4) or (5) "transition" holes located at the bottom of the butterfly. This is one of the reasons that these carbs work so well. A Linkert feeds fuel through a thin slot vs. (5) small holes, so the Keihin issues (5) small mists of fuel spray, instead of a bunch of raw fuel droplets. This better atomization changes everything! Modern fuel is designed for fuel injection systems to spray fuel as a fine mist under high pressure. Unlike older fuels that readily atomized more easily in carburetors, new fuel comes out as droplets that go unburned in the combustion chamber. It will leave black deposits on the plugs, and tailpipe giving the rider the impression that the bike is running rich, when, in fact, the bike is actually running leaner, and hotter! The poor atomization of the Linkert carbs leads to lean conditions that create detonation, heat, loss of power, cylinder distortion, and poor milage. The Keihins (5) small holes properly atomize todays fuel, and unlock alot of power hidden within the Indian! I have noticed that some of these carbs have (4) holes, and most have (5). I don't think that there would be much difference in performance. The Main Jet is primarily sized for top end performance, but also feeds the mid-range mixture needle in the slide. The mid-range needle, which meters fuel from low speed through high speed raises to allow more fuel as the slide is lifted. The slide is lifted only when there is a vacuum signal strong enough to do so based on air flow through the carb. Therefore, the throttle grip opens the carb's butterfly, but the needle only raises to the correct position based on the engines demand. The needle's design, and position here are very critical.The throttle can never be opened too much, or too soon at any time, because the slide will only raise if there is sufficiant demand from the engine. Complicated stuff! The Europeans have used CV carbs for many years on their cars with great success.

Why Use A CV Carburetor?

Why are CV carbs superior to the Linkert? For one, the Linkert is a large "Lawn Mower" styled carb that delivers fuel, and air with little consideration for optimum atomization. Yes, it will start, and run,and is somewhat adjustable, but there is alot of extra torque and horsepower inside your motor being held hostage by the Linkert (not to mention how hard a Linkert is on cylinders, and pistons from heat). I am not advocating "getting rid of" your Linkert. On the contrary, mine is now proudly displayed on the wall with all of it's modern improvements like Cotton's new black plastic float, and Viton tipped needle and seat. I have mine preserved for the future for originality if I wish to take the bike out of hard duty service and go back to original. My kit is a simple bolt on which only requires shortening the throttle cable by 5 inches, which is easy to replace later. The original air cleaner back must be modified, or replaced with one of my new replacement units to fit the new carb.

CV carbs can automaticly adjust mixture for changes in elevation (a very wide range). As you go up in altitude, you are in thinner air. So, at the same RPM and load, there is less air flowing through the carb, which causes less vacuum at the bottom of the slide, and the needle rises less. This leans the carb over a broad RPM spectrum.

Once installed, spark fully retarded, and the choke set, 3-4 hard pumps with the throttle, the bike always starts on the first kick. The choke is slowly turned off, and spark advance is slowly added as the bike warms up. The Chief now sounds like a modern bike, running smoother with more authority. The bike also starts much easier when warm. Fuel milage is increased. On my own 1947 80" Bonneville, I achieved 53 MPG at 65 MPH over 150 miles! The power increase is incredible. The bike sounds completely different when "on-power" also. The bike never hesitates, or misses, ever, and the revs are much quicker. This carb makes so much extra torque, and power that the stock gearing (usually 23 tooth) can be increased up to the larger 26 tooth like the big 80's for higher cruise speeds at lower RPMs. For those looking for extra performance, and have stepped up to the 80", and installed hotter cams, this carb is a must. On a 1951 80" Bobber with "Ollie" cams, we increased the "Seat of the Pants" feel of power by around 50% over the Linkert! Dyno tests showed that this bike makes the same power as a stock H-D Evo motor.

Ease of maintenance and operation is also a virtue. These carbs are very easy to work on, and parts are available at any H-D dealer. While riding, there are no needles to turn. Because these are CV carbs which meter fuel based on flow, they are somewhat altitude compensating. The flow induced vacuum under load lifts the slide, which carries the mixture needle for mid-range. As you go up in elevation the air thins out, the flow through the carb slows, and the needle doesn't rise as high yielding the necessary leaner mixture. My own tests have shown good results from the same settings when ridden in Borrego Springs, Ca. (below sea level), as well as in the Colorado Rockies above 11,000 feet. Proper mixture yields maximum power without an increase in heat, providing that the "Best" ignition timing is set. The better atomization requires much less full advance, so with less heat comes less detonation, and less top end wear. Timing is crucial, and will be covered here later. Full retard to TDC, and full advance to 23-24 degrees.

Important-Read Each Catagory Below for Full Details!

Click Here for: Full Details On Keihin CV Carb Kit.

Click Here for: Keihin 41mm CV Carb Info.

Click Here for: Chinese CV40 Carb Mods.

Click Here for: Step-By-Step Installation Instructions.

Click Here for: Air Cleaner Options.

Click Here for: Yost "Power Tube" Jet Kit Info.

Click Here for: Do-It-Yourself Carb Mods.

Click Here for: Tuning the CV, and Timing Specifications.

Click Here for: Dyno Charts, Tech Talk, and Tuning for Power.

Click Here for: Ordering Info.


James R. Mosher