(From the J&P Cycles Archive)

Regardless of whether your motorcycle uses a carburetor or fuel injection, the correct air/fuel mixture is important. Any changes to either the intake tract or exhaust system can impact the air/fuel mixture.

Whenever you make a change in the airflow through the engine, changes can occur in the fuel/air ratio on your bike. Bad conditions that we need to correct are running either too lean or too rich. Running lean can burn pistons and score cylinders. Running rich fouls plugs and makes a bike run bad.

Changing the air cleaner or the pipes changes the mixture. Changing baffles also changes the mixture. With a carbureted bike, the fix is to just change the jets. With a fuel-injected bike it's easier, the fix is to install a Power Commander.

Carburetor jetting is very complex. Factors such as humidity, altitude, and air temperature all affect the ratio. With a carbureted bike, if you have a dyno and an exhaust gas analyzer, the adjustments are very easy and straightforward. To change to the correct jet, you must understand where the jets are effective. We will discuss the CV carb that came on Big Twins from 1990 through 2004 and XL's from 1988 to 2004. On a CV type carb, the pilot jet and idle mixture screw control throttle position is up to 1/4 open. The midrange is controlled by the thin needle position in the slide, and 3/4 throttle and up is controlled by the main jet. The correct air/fuel ratio is about 14.7 to 1, or 14.7 parts air to one part fuel. Without an exhaust gas analyzer, you will need to make an adjustment and read the spark plugs (check plug color) to determine the effect. The desired color is a very light tan to chocolate brown.

Here is a basic procedure to determine what jet to change. First, change the components such as pipes or air cleaner and get yourself three or four sets of new plugs, gapped and ready to install. We need to have access to the idle mixture screw, so we need to drill out the plug covering it. Once this is done, screw it in until it bottoms lightly, then back out 11/4 turns. Put a piece of masking tape around your throttle. Pick an easily seen reference point on your switch housing and mark on the tape where the zero (0) throttle position is. Open the throttle wide open, and mark on the tape the full throttle point. Halfway between these marks mark again, and divide the 2-1/2's in 1/2 again. When you are though, you will have a mark at 0, 1/4, 1/2, 3/4 and Wide Open Throttle on the throttle sleeve.

Next, start the bike and warm it up so that it idles without the use of the enrichner. Once that's done, while wearing a set of mechanics gloves, remove the spark plugs and install a new set. Take the bike out and ride it for about 10 to 15 miles at 1/4 throttle if possible.

Cleaning and adjusting your carb can be tricky but is something you can do yourself with a few pointers and patience. 

Things get a little complex now, so pay attention. We need to do what's called a "plug chop." This is where you pull the clutch, kill the ignition, and pull to the side of the road (carefully of course) WITHOUT ALLOWING THE BIKE TO IDLE. Still using your mechanic gloves, remove a plug and look at the color. If they are black and sooty, you are running rich on the pilot jet and need to reduce the amount of fuel by installing a smaller jet. If they are clean white, you are lean and need to give the engine more fuel by using a larger pilot or turn the mixture screw out a 1/4 turn. If they are brown no change is required. To aid in tuning, when you go in with the mixture screw, you get a leaner mixture. Out you richen the mixture. As a general rule of thumb the idle mixture should be 1-1/4 to 1-1/2 turns out when you have selected the right pilot jet. Repeat your test with fresh plugs for 1/2 throttle. In order to make a change here, we can add fuel only with a stock needle by shimming with tiny washers under the needle. Every shim you add richens the midrange. The Dynojet tuner kits are equipped with a needle that has grooves in the end to adjust the position with a clip. Remember the higher on the needle the clip is the leaner the midrange becomes.

Install the appropriate jet you have decided upon (or make needle change), and a new set of plugs and redo the test. Repeat this until you have the correct light chocolate appearance.

The main jet is your cruising jet and needs to be tested above 3/4 throttle. It requires a nice stretch of open road, or better yet a drag strip. Repeat the same type of test as before at 3/4 throttle and full throttle, doing your plug chop. Make changes accordingly. An additional test that is handy is to get up to, fourth gear at about 4,000 rpm. Open the throttle all the way then immediately let off the throttle about 1/8. If the engine slows just a bit, the jet is close, if it seems to pick up speed or rpm, the main jet is too lean. If it hesitates or stumbles, the main is too rich. Change jets and test again.

After you have the jets close, I just ride about 50 to 75 miles observing the performance of the bike at different speeds. Poor acceleration, pinging, knocking, surging and popping or spitting through the carburetor suggests you're still lean. Black smoke, sooty exhaust, smell of unburned fuel and a rough idle suggest too rich. When I get back from the ride, I pull the plugs once more and see what I've got. If they are tan, light chocolate and the bike runs good, I'm done.

With the Power Commander, things are slightly different. With a PC you will need to access the Power Commander website and select the map that is closest to your combination. This must be downloaded into your Power Commander. If the bike does not run well, determining your needs are very similar to the tests outlined above. Mark your throttle, do your plug chop, read the plugs. Unfortunately it is a little more complex to make a change. Accessing the map in the Power Commander, you can change cells in the map to richen and lean. This is a simplified version of tuning the Power Commander, but it should give you an idea of what's required.