Keihin CVK40 (CV40, CV)carburetor tuning tips

The following article excerpts are reprinted with permission. Original article credit goes to Direct Parts / CCP by Direct Parts.

Carburetor tuning basics:

Keep in mind that motorcycle carburetion is a compromise and is never perfect. The Basic circuits are the Enrichener circuit: Aids in cold starting and warm-up. Accelerator Pump: Aids in starting and off idle transition. Idle & Low Speed circuit: Controls idle, and helps transition to Midrange. Midrange circuit: Controls steady throttle and light acceleration. High speed circuit: controls hard acceleration and wide-open throttle. All circuits overlap, but each one has a primary function. Idle (throttle closed): controlled by slow speed jet and mixture screw. Midrange (steady throttle and light acceleration): controlled by needle. High Speed (Wide Open Throttle): controlled by main jet. Tuning: Base Line Tune each circuit for it's primary function. Idle & Low Speed circuit, control 100% of idle and about 25% of right off idle low speed. The primary function of this circuit is the Idle. There is nothing else controlling fuel here. Wide-open throttle hard acceleration, when the slide/needle is all the way up the main jet is your primary controller of the fuel. The needle controls fuel for everything in-between. The needle starts to fuel right off idle with light throttle. As soon as you start to open the throttle air velocity increases under the slide, and fuel drawn from around the needle, even before the slide lifts. Bottom line, set the idle jet and fuel mixture screw for the best idle. Set main jet for best wide-open throttle. The needle will control mid-range, steady (cruise) and light throttle.

Tuning: Ride-ability problems. When you experience motorcycle ride-ability problems that are jetting* related, try to duplicate the problem in all of the gears, and under different temperatures (engine and/or air). Remember that air temp can vary greatly from morning to afternoon, and it can take up 20 miles of riding for the bike to reach operating temperature. Decide which circuit needs adjusting.
- Idle or idle and light throttle: Slow speed (pilot) jet/mixture screw.
- Steady throttle and/or light acceleration: Needle.
- Wide open throttle hard acceleration: Main jet.
Decide if you are rich or lean. Worse hot: Symptom of rich. Worse cold: Symptom of lean. Worse in higher gears: Symptom of rich. Worse in lower gears: Symptom of lean. Make your adjustment, and then try to duplicate the problem again. It is important to only make one adjust at a time. Instead of changing the main jet it may be easier to test rich or lean first, by removing the air filter if you think it is too rich or duct taping ¼- ½ the air filter surface area if you think it's too lean. Note: Doing the above test may cause a problem in a different throttle area, ignore this and focus on the problem area you are working on.

*Do not confuse other motorcycle problems, including carburetion problems with jetting! Using this technique and the plug chop method you can get that Keihin CV on your Harley or Kawasaki working as good or better than the high buck options.

Carburetor tuning using the Plug Chop method

Note that tuning by using the plug chop method can be an acquired skill that takes practice. As an alternative you could install a Tuners Kit which has been designed for your specific model.

The most commonly asked question about carburetor and Fuel Injection is "How do I know if my jetting is correct? A typical question might go; " I just added a new motorcycle exhaust system to my bike. Do I need to change the jetting? My exhaust color seems to be fine." Or, "I don't have access to a Dyno, how do I tell if my jetting is correct?" Long before Dynamometers became common, we used to do jetting using the "plug chop" method. We will discuss it and some general jetting rules here. From the factory, your motorcycle came with standardized EPA mandated lean jetting and restrictive intake and exhaust. In some locales, this jetting is so lean it must be changed to prevent damage to the engine. The standard jetting has varied over the models of carburetors and model years. All of the motorcycle part information below is based on your engine intake system being properly sealed and no other problems exist. An intake leak will throw spark plug readings way off. One of the most common problems is the off idle "pop" and backfire at slow engine speeds. This is usually caused by the idle mixture being excessively lean and is easily corrected by correct jetting and / or the use of a jet kit. The ignition timing needs to be correct and the ignition system must be functioning correctly. A weak or poor spark will drastically alter your plug readings. We like to do our jetting in two stages. First, we need to get the idle and low speed jetting correct and then we work on the high speed or main jetting. In our example, we will discuss the Keihin CV (CVK40, CV40) used since 1989 on Sportsters and 1990 on Big Twins. The adjustments may differ for S&S "E", HSR Mikuni and others such as the Quick Silver from Edelbrock. Before you start, have a collection of fresh unused plugs gapped and ready to use. We will need at least three new sets for this test. We use Champion plugs for plug chops. You can get them at local motorcycle parts stores for far less than the OEM plugs. EVO Big Twin use RN12YC. EVO XL and TC88 use the RA8HC. On the CV, adjust the idle mixture while idling. Closing the idle mixture screw slowly should cause the idle to become rough. Slowly turn the screw out until the engine again idles smoothly. Add approximately 1/8 to ¼ turn more. If closing the screw makes no difference in idle speed or smoothness, you will have to use the next smaller pilot jet. Until the 2000 model year, most Big Twins have a #42 as the stock pilot jet. 2000 models use a #45 stock. In some cases, depending on the bike setup and altitude, a #45 may actually be too large. An aftermarket #44 pilot jet works very well under most conditions. If you have to turn the screw out more than 3 turns, consider increasing jet size to the next larger pilot. The JCP EZ-Just idle needle for Keihin CV 40 (CV40, CVK40, CVK44, CVK34) comes in handy here as it allows you to adjust the mixture without tools.

After setting the idle speed and idle mixture correctly, change to a fresh set of plugs. We highly recommend using some anti seize on the threads to prevent galling. After installing the plugs, you want to ride around at part throttle gently accelerating but keeping below ½ throttle. You want to stay on the low speed circuit as much as possible. After about one mile, pull in the clutch and kill the engine. Remove the sparkplugs. Look at the insulator from the tip down as far as you can see. We are not concerned with the grounding tip or the base of the plug. We are looking for an off white, eggshell to a light Navajo like color. Note: If you are getting what looks to be a tan color, you are running rich. Tan is not possible since there is no lead in the fuel and that is where the tan color came from. The brown to chocolate brownish colors are indications of being rich. The rear plug should read slightly richer than the front and you want to jet off of the front cylinder readings. The reason for the difference is inherent in the design of the 45° V-Twin and the intake. If you are reading too rich, you may need to lean out the pilot jet or close the screw slightly. If you are reading very white or no color at all you will need to open the idle mixture more or go up one size. If the plug reading is good, we are set to do the main jetting. To do the main jet you need a place where you can accelerate at wide-open throttle through at least third gear, preferably fourth gear safely. The procedure is simple. After riding to the location, change to a fresh set of plugs. Start the engine and immediately accelerate at wide-open throttle as described above. Do not idle or use motorcycle part throttle. After accelerating, pull in the clutch; hit the kill switch and coast to a stop. Remove the plugs and examine as before. If the plugs are reading very white of no indication at all, you are lean and need to go up one size on your main jet. If you are reading a tan or darker, you will need to go down at least one size depending on how dark the plug reads. You will need to repeat this procedure until the plug readings are correct. This is why we suggest using inexpensive plugs. You can change back to the more expensive ones later. After getting your plugs reading right, your jetting is spot on and you will be producing the most power and getting the most mileage for your setup.

Note: On older Keihin CV's and all Keihin Butterfly carburetors check the inlet fuel elbow for cracks or leaks. The injection molded plastic elbow breaks easily and can cause a dangerous fuel leak. If your elbow is cracked I suggest that you play it safe and replace it with a Hi-Flow brass fuel inlet elbow for all Keihin CV and Butterfly carburetors. The H-Flow elbow fits the CVK34, CVK40, CVK44 Keihin series carburetors.