CV Carburetor Jet Adjustment/Trouble Shooting

CV Carburetor Jet Adjustment

Our most common 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 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. Drag Specialties provides a #44 pilot jet, which 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. A Creative Cycle Products idle mixture needle (CP005) 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 very, very light tan. 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 to 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. With todays fuels that are missing the lead, it's difficult to get a reading from the plugs at all.

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.

Fine tuning: When you are getting close on the main jet you can fine tune with the needle if yours has a clip raising the clip lowers the needle and leans out the mixture. Lowering the clip richens the mixture. If your carburetor is not equipped with a clip, thin washers can be used to do fine tuning if needed.


Here's a quick test that will tell you if you have to fat of a jet in the top end....
Run the bike through the gears into third gear and run third up to 60-70 MPH full throttle, then chop the throttle about 1/8 to 1/4 back and see if the bike either surges ahead or stumbles then recovers.

If it surges ahead, your jetting is lean
If it stumbles and recovers , your jetting is rich
If it makes no difference, you're pretty close.

Quick and easy, pretty accurate too.